Answer
The charge on each sphere is $~-5.0\times 10^{-7}~C$
Work Step by Step
We can find the magnitude of the charge $q$ on each sphere:
$F = \frac{k~q~q}{r^2}$
$q^2 = \frac{F~r^2}{k}$
$q = \sqrt{\frac{F~r^2}{k}}$
$q = \sqrt{\frac{(0.036~N)(0.25~m)^2}{9.0\times 10^9~N~m^2/C^2}}$
$q = 5.0\times 10^{-7}~C$
Since the magnitude of the charge on each sphere is $5.0\times 10^{-7}~C$ and the charge is negative, the charge on each sphere is $~-5.0\times 10^{-7}~C$
