We are to prove that the angles with the green arcs are equal, ∠PXR = ∠QPR given that the angles with the red arcs are equal, ∠RPX = ∠Q We use the fact that the three internal angles of a triangle have sum 180°, For ΔPQR, (1) ∠QPR + ∠R + ∠Q = 180°. Therefore, (2) ∠QPR = 180° - ∠R - ∠Q For ΔRPX, (3) ∠PXR + ∠R + ∠RPX = 180°. Therefore, (4) ∠PXR = 180° - ∠R - ∠RPX Since we are given that ∠RPX = ∠Q, the right sides of (2) and (4) are equal, so their left sides are also equal. Therefore, ∠PXR = ∠QPR Now you can write that up in a two-column proof. Edwin