Answer
$V = 850~V$
Work Step by Step
We can find the total energy that was supplied:
$U = P~t = (6500~W)(5.0\times 10^{-3}~s) = 32.5~J$
We can find the voltage:
$U = \frac{1}{2}CV^2$
$V^2 = \frac{2U}{C}$
$V = \sqrt{\frac{2U}{C}}$
$V = \sqrt{\frac{(2)(32.5~J)}{90\times 10^{-6}~F}}$
$V = 850~V$
