Question

He measure of angle AFB = 78°

Answer 1

It’s angle AFE because its the one in the same line as AFB and since the line is 180 degrees that means they add up to 180

Answer 2

Angle AFE is supplementary to angle AFB because they are both along the line with points EFB. When two angles are along a straight line, they equal 180 degrees. This is also what the definition of supplementary is.