b Notice that this is a triangle with two congruent sides. Therefore, this is an isosceles triangle which means its base angles are congruent, a = b. With this, we can either replace b with a or a with b.
By the Triangle Angle Sum Theorem, the sum of a triangles three angle measures is 180^(∘).
a+a+90^(∘)=180^(∘)
Let's solve the equation for a.