Hello!
As you can see, the angles are congruent. These kind of angles are called vertical angles.
Vertical angles are angles opposite of each other. They always measure the same.
In this pair of vertical angles, one of them measures 60°. That means the other angle also measures 60°
60 = 45 + x
Subtract 45 from 60 to find x60 - 45 = 15
