Take the speed of light as 3 x 10^8 m/s. Fully converting 2 kg of matter to energy gives E = 1.8 x 10^n joules. Enter the value of n.

17

$c^{2} = 9 \times 10^{16}$, so $E = 2 \times 9 \times 10^{16} = 18 \times 10^{16} = 1.8 \times 10^{17}$ J. The renormalisation from 18 to 1.8 bumps the exponent by one, a step routinely dropped, giving 16. The enormous size of the number is why a tiny mass powers a reactor.