M2 = a°, m 3 = b°, m 4 = a b a + b a - b (see picture)

The answer is a+b. Since a triangle is equal to 180 degrees, and you know that angle 2 = a degrees and angle 3 = b degrees then you know that 180 degrees minus angle 2 and angle 3 = angle 1.

It is also known that angle 1 and angle 4 added together will be 180 degrees. Since before we already found out that angle 1 = 180 - angle2 + angle3, we know that angle4 = angle2+angle3