SAT Question 9adb86ed: Circles answer and explanation with Desmos solution

Choice B is correct. Since it's given that $P$ is the center of a circle with a radius of $9$ inches, and that points $Q$ and $R$ lie on that circle, it follows that $PARS{E}_{E}RROR:Entit{y}^{\prime }mac{r}^{\prime }notdefined,line1,column139(<string>,line1)$ and $PARS{E}_{E}RROR:Entit{y}^{\prime }mac{r}^{\prime }notdefined,line1,column139(<string>,line1)$ of triangle $PQR$ each have a length of $9$ inches. Let the length of $PARS{E}_{E}RROR:Entit{y}^{\prime }mac{r}^{\prime }notdefined,line1,column139(<string>,line1)$ be $x$ inches. It follows that the perimeter of triangle $PQR$ is $9+9+x$ inches. Since it's given that the perimeter of triangle $PQR$ is $31$ inches, it follows that $9+9+x=31$, or $18+x=31$. Subtracting $18$ from both sides of this equation gives $x=13$. Therefore, the length, in inches, of $PARS{E}_{E}RROR:Entit{y}^{\prime }mac{r}^{\prime }notdefined,line1,column139(<string>,line1)$ is $13$.