$\begin{align}  & \angle PQT=\angle QRT={{70}^{\circ }}\text{    }\!\!\{\!\!\text{ Base angles of Iss}\text{. }\vartriangle \text{ }\!\!\}\!\!\text{ } \\ & \angle PTQ={{180}^{\circ }}-\angle PQT-\angle QRT\text{   }\!\!\{\!\!\text{ Sum of }\angle s\text{ in a }\vartriangle \text{ }\!\!\}\!\!\text{ } \\ & \angle PTQ={{180}^{\circ }}-{{70}^{\circ }}-{{70}^{\circ }}={{40}^{\circ }} \\ & \angle STR=\angle PTQ={{40}^{\circ }}\text{   }\!\!\{\!\!\text{ Vertically opposite angles }\!\!\}\!\!\text{ } \\ & \left| ST \right|=\left| SR \right|\text{    }\!\!\{\!\!\text{ Radius of a circle }\!\!\}\!\!\text{ } \\ & \angle TSR=\angle SRT=x\text{   }\!\!\{\!\!\text{ Base angles of an isso }\vartriangle \text{ }\!\!\}\!\!\text{ } \\ & \angle STR+\angle TSR+\angle SRT={{180}^{\circ }}\text{ }\!\!\{\!\!\text{ Sum of }\angle s\text{ in a }\vartriangle \text{ }\!\!\}\!\!\text{ } \\ & \text{4}{{\text{0}}^{\circ }}+x+x={{180}^{\circ }} \\ & x={{70}^{\circ }} \\ & \angle SRT=\angle PRS={{70}^{\circ }} \\\end{align}$