[tex]\textit{using the pythagorean theorem} \\\\ c^2=a^2+b^2\implies \sqrt{c^2-a^2}=b \qquad \begin{cases} c=hypotenuse\\ a=adjacent\\ b=opposite\\ \end{cases} \\\\[-0.35em] ~\dotfill\\\\ \sqrt{20^2-16^2}=PQ\implies 12=PQ \\\\\\ PS=\sqrt{12^2+5^2}\implies \boxed{PS=13}[/tex]
Check the picture below.
