Triangles must equal 180 degrees. The given 50 means the rest of the triangle is equal to 130. I believe because of the Side-Angle-Side theorem of triangles (if my knowledge of proofs from 3 years ago is still with me), the two remaining angles are equal to each other. Let's call them X. 2x = 130. Divide both sides by 2. 130/2 = 65. So x=65
