Both triangles are isosceles. The inside one is also a right triangle. Since the big triangle is isosceles and the two angles add up to 120 degrees, they also have to be equal, meaning ∠P≅∠R=60 degrees. The inside triangle is also isosceles, meaning that m∠a=60°-b, which 60°-b representing the measure of the last angle in the smaller triangle. Since ∠a and the third angle are congruent, they each have to equal 45°. 60°-45°=15°. The measure of ∠b=15°.
