All three angles in an equilateral triangle are the same (60 degrees). The problem tells us that the angles A and B are bisected. This means that they are split into two 30 degree angles. The two lines meet in the middle of the triangle at point F.
Now- those two lines make a new triangle (along with side AB of the big triangle). This triangle has two angles of 30 degrees each, and since the sum of the angles in a triangle must be 180, the third angle (AFB) is 120 degrees. (180-30-30)
