MBI Business Centers Inc., the Rockville-based computer-store chain formerly known as The Math Box Inc., yesterday announced that it had agreed to be acquired by Businessland Inc. of San Jose in a stock swap valued at $75 million.

The purchase dramatically expands the number of Businessland stores on the East Coast while highlighting the trend toward mergers, acquisitions and consolidation that has been sweeping through the troubled computer retail market.

"This is two industry leaders joining forces," said Alex. Brown & Sons Inc. analyst Franklin L. Morton, who contrasted this merger with the "distress sale" nature of other computer retail acquisitions. "In the case of both companies, they're getting rid of their major competitor."

The acquisition, negotiated over the last week and approved by MBI's board Friday, would create a computer retail giant with $600 million a year in revenue and nearly 150 stores nationwide.

The transaction calls for an exchange of stock, with MBI holders receiving 1 1/2 shares of Businessland for each MBI share. The deal values MBI stock at $15 a share -- 25 cents above its historic high and $1.25 above yesterday's close of $13.75, up 50 cents. Businessland stock closed yesterday at $10.50, up $1.

When the deal is formally concluded in August, it will turn a $400,000 investment in four stores in 1981 into $75 million, with a personal windfall for MBI Chairman Avner Parnes approaching $4 million. Parnes will become vice chairman of Businessland.

With roughly 500 employes, MBI has 36 computer stores situated primarily on the East Coast. The company's 1985 sales are estimated at $118 million, with roughly a $3 million after-tax profit. In April, MBI acquired Computer South's chain of 11 computer stores in North and South Carolina.

MBI also holds a valued General Services Administration contract to provide the federal government with personal computers through a set of special stores. That contract accounts for nearly one-quarter of MBI's revenue and is up for renewal in June.

Parnes and Businessland Chairman David Norman said that contract renewal was not a factor in the merger.

Businessland, which has 1,500 employes, had revenue of $350 million and net income of $3.56 million in 1985. There are 69 Businessland centers in 28 major metropolitan areas. In April, Businessland said it would acquire United Telecommunications Inc.'s 36 AmeriSource computer centers, most of which are in the Midwest. That deal is expected to close in June.

"What IBM is in manufacturing, we will be in distribution," said Parnes, who predicted the new company will enjoy "$1 billion in revenue within two years."

Parnes said that the combined strength of the two companies could improve their bargaining power with key computer suppliers ranging from International Business Machines Corp. to Apple Computer Inc.

The two companies have enjoyed relatively high profit margins in a crowded and competitive industry mainly because of their exclusive focus on marketing to business users rather than seeking home-com-puter customers. The companies also have developed extensive consulting, technical and sales expertise to sell to businesses on-site rather than just at the stores.

"Both of us only address the business marketplace," Norman said. "There's no question that that's where we excel."

Norman conceded that there might be some slowing in sales growth but said, "Our concentration now is really on the bottom line. . . . We had to be positioned as a national company," and the MBI acquisition was crucial to that goal.

"Both MBI and Businessland have taken enormous bites to chew," said Seymour Merrin, a computer retail analyst with the Gartner Group. "It certainly expands Businessland's network rapidly. The question is, do they have the management to manage it?"

MBI President Armen Manoogian will become a Businessland vice president and director.

Merrin said that MBI is being purchased for a premium price and "it is a magnificent move for Avner Parnes. I quote that magnificent philosopher Bernard Baruch when he was asked how he made his fortune -- 'By selling too soon,' he replied."