You can tell that this triangle is an isosceles triangle as both sides are the same length. In an isosceles triangle, the bottom two angles are always equal. The angles in a triangle always add to 180 degrees. You are given the fact that the top angle is 48 degrees. If you do 180 - 48, that will give you 132 degrees, which is the sum of the bottom two angles. If you divide that by 2, you will get 66 degrees, which is one of the angles.
