Check the picture below.
[tex]\textit{using the pythagorean theorem} \\\\ c^2=a^2+b^2\implies c=\sqrt{a^2 + b^2} \qquad \begin{cases} c=\stackrel{hypotenuse}{RP}\\ a=\stackrel{adjacent}{15}\\ b=\stackrel{opposite}{8}\\ \end{cases} \\\\\\ RP=\sqrt{15^2+8^2}\implies RP=\sqrt{225+64}\implies RP=17[/tex]
