[tex]I = \sqrt{\frac{P}{R}}[/tex]
Start by squaring both sides. This ensures that we can get rid of the square root, and this makes it easier for us to deal with.
[tex]I^{2} = \frac{P}{R}[/tex]
Now, multiply both sides by R, to get rid of the fraction.
[tex]I^{2}R = P[/tex]
Now, simply divide both sides by the square of I.
[tex]R = \frac{P}{I^{2}}[/tex]
