Triangle ABC is similar to triangle PQR.

So, because of the way the triangles are named:

AB is to PQ as AC is to PR.

9 is to 15 as 12 is to PR

\(\frac{9}{15}=\frac{12}{PR} \\~\\ PR*\frac{9}{15}=12 \\~\\ PR=12/\frac{9}{15} \\~\\ PR=12*\frac{15}{9} \\~\\ PR=20\)