$\begin{align}  & \angle PQR=\angle QRS=y\,\,\,\,\,\{alternate\text{ angles }\!\!\}\!\!\text{ } \\ & x+y={{180}^{\circ }}\text{    }\!\!\{\!\!\text{ sum of }\angle \text{s on a straight line }\!\!\}\!\!\text{ } \\\end{align}$