So we have a problem with two unknows and one equation. We have to express one over the other like this: 7a - 2b = 5a + b. First we separate one kind on the left side and the other kind on the right side: 7a - 5a = b + 2b. Then: 2a = 3b. Now we divide both sides by 2 and get: a= 3b/2.