Note that SQ is shown with a break, but then it would not be a line, so one must assume SQ is a single straight line.
Draw line ST. Now you have triangles STQ and RTS. For these two triangles you have RS = QT and RT = SQ (both given) AND ST = ST. So by SSS (side-side-side), you have triangle STQ congruent to triangle RTS. For congruent triangles, corresponding angles are equal.
