Solve the equation w=Cr^-2. Solve for r
$\displaystyle w=Cr^{-2}$
This is equivalent to:
$\displaystyle w= \frac{C}{r^2}$
Multiply both sides by $\displaystyle r^2$:
$\displaystyle wr^2 = C$
Divide both sides by $\displaystyle w$:
$\displaystyle r^2 = \frac{C}{w}$
Square root both sides:
$\displaystyle r = \pm\sqrt{\frac{C}{w}}$