THE DECISION by Secretary of Transportation Brock Adams to free about a billion dollars in highway funds for Metro construction doesn't solve Metro's finacial problems. But it does provide a formula for solving them. With some hard work - and a little luck in Congress and with the state and local governments - the Washington area should have its full, 101-mile subway system by 1990.

The three-year dispute over financing, which the agreement between Secretary Adams and Metro's board partially resolves, has already slowed down Metro's construction and added to its cost. But the issues have nbeen critical - how much more money will the federal government put up, how will the local governments pay their share, and which lines will be built first. Only the last of these has been fully settled.

The money Mr. Adams has agreed to release will let Metro keep building for the next four years. After that, it will be up to the Congress. The House is scheduled to vote Monday on the Stark-Harris bill, which provides an additional $1.7 billion in matching funds over the six years beginning in 1982. If Congress passes that bill - as it should - Metro's future, for the first time in years, will rest completely with local officials.

In this three-way arrangement to finance Metro properly, the local governments are obligated to find a reliable source of funding not only to match the new federal construction money but also to pay off some of the bonds Metro floated years ago. That will not be easy, especially in Virginia where the state government gives highways priority over subways.

It is unfortunate it took so long for Metro and local officials to persuade Secretary Adams to release the funds from cancelled interstate highway projects. Neither the time nor money lost can be reclaimed. Both sides were remarkably stubborn, but the long delay did not produce a good financing package.

There is no such off-setting accomplishment, or even goal, to explain the Carter administration's failure to support the Stark-Harris bill. That bill is the logical next step in the process in which Congress has been deeply involved for a decade and a half. Its passage may well be necessary to the completion of Metro, and completion is clearly necessary to the well-being of the Washington area. While Secretary Adams understands as much, the White House has been unable, or unwilling, to understand. As a result, President Carter - alone among all residents of the White House since Metro was conceived - has refused to come to its aid when a critical problem arose.