Because of the Isosceles Triangle Theorem, angle B and angle C have the same measure. If angle B is 75, then so is angle C. By the Triangle Angle-Sum Theorem, all the angles of a triangle have to add up to equal 180 degrees. That means that angle A = 180 - 75 - 75 which is 30 degrees. Therefore,
Angle A = 30 degrees. By the substitution property of equality,
