Answer
$U = 1.20\times 10^{-10}~J$
Work Step by Step
In part (a), we found that the potential difference across the plates is $16.0~V$ after the plates are pulled apart.
We can find the final stored energy:
$U = \frac{1}{2}CV^2$
$U = \frac{1}{2}(\frac{\epsilon_0~A}{d})~V^2$
$U = \frac{(8.854\times 10^{-12}~C/V~m)(8.50\times 10^{-4}~m^2)(16.0~V)^2}{(2)(8.00\times 10^{-3}~m)}$
$U = 1.20\times 10^{-10}~J$
