Triangles CPA and CPB are both right triangles. They share a leg, so that leg in one triangle is congruent to that leg in the other triangle. We are given that PA is congruent to PB by the hash marks on the diagram. Thus two legs and an included angle are congruent between the triangles.
... ∆CPA ≅ ∆CPB by the SAS postulate
Then side CA ≅ CB = 15 in, because corresponding parts of congruent triangles are congruent (CPCTC).
