$\triangle$ABC $\sim$ $\triangle$QRP
$\frac{\operatorname{ar}(\Delta \mathrm{ABC})}{\operatorname{ar}(\Delta \mathrm{PQR})}=\frac{9}{4}$
AB = 18 cm, BC = 15 cm
$\frac{A B}{Q R}=\frac{B C}{R P}=\frac{\sqrt{9}}{\sqrt{4}}=\frac{3}{2}$
$\frac{15}{R P}=\frac{3}{2}$ $\Rightarrow$ RP = $\frac{15 \times 2}{3}$ = 10 cm
अतः, PR = 10 cm