(This will probably be a way more complex explanation than you need but it’s what I did.)
Okay so you know B=B because they’re both right angles. You know A=D because of the isosceles triangle theorem- if two sides of a triangle are congruent, then the angles opposite those sides are congruent. That leaves C=C. If B=B and A=D and all triangles add up to 180 degrees, C and C need to be the same.
