Answer
$U = 4.52\times 10^{-11}~J$
Work Step by Step
We can find the initial stored energy:
$U = \frac{1}{2}CV^2$
$U = \frac{1}{2}(\frac{\epsilon_0~A}{d})~V^2$
$U = \frac{(8.854\times 10^{-12}~C/V~m)(8.50\times 10^{-4}~m^2)(6.00~V)^2}{(2)(3.00\times 10^{-3}~m)}$
$U = 4.52\times 10^{-11}~J$
