Ok, so the measurement of all the angles together has to equal 360 degrees. So the angle directly across from 100 is also 100 (because they are vertical angles) which equals 200. That means you have 160 degrees left to figure out. Since you have to find the measurement of the other TWO angles, just divide 160 in half and then you have the measurement for angle b.
The answer is 80 degrees.
Sorry if this is a long answer. I hope it helps though.