#### Answer

84 degrees

#### Work Step by Step

We know that angle Q is equal to $b-a$, meaning that $b-a$ is equal to 42 degrees. Because all triangles add to 180 degrees, we obtain:
$ 180 = 180 - 2b + 2a + M \\ 2b - 2a = M \\ 2(b-a) = M \\ M = 2(42) = 84$