Answer
$7.2\times10^{19}J$.
Work Step by Step
The 400 kg of antimatter annihilates with 400 kg of matter. The energy release is now calculated.
$$E =mc^2=(800kg)(3.00\times10^8 m/s)^2=7.2\times10^{19}J$$
This is approximately 72 percent of the US annual energy use.
