$\begin{align}  & P=\sqrt{\frac{r{{s}^{3}}}{t}} \\ & \text{Square both sides} \\ & {{P}^{2}}=\frac{r{{s}^{3}}}{t} \\ & \text{multiply both sides by }\frac{t}{{{s}^{3}}} \\ & r=\frac{{{p}^{2}}t}{{{s}^{3}}} \\\end{align}$