Answer
$P=25 I^2$.
Work Step by Step
The given formula is
$I=\frac{1}{5}\sqrt{P}$
Multiply both sides by $5$.
$5\times I=5\times\frac{1}{5}\sqrt{P}$
Simplify.
$5 I=\sqrt{P}$
Square both sides.
$(5 I)^2=(\sqrt{P})^2$
Simplify.
$25 I^2=P$.