Answer
6.0 kg
Work Step by Step
Given/Known: $Q= 2.0\times10^{6}J$,
$\Delta T=(100-20)^{\circ}C=80^{\circ}C$ and $c=4186\,J/(kg\,^{\circ}C)$
Equation to find m:
$c=\frac{Q}{m\Delta T}$ or $m=\frac{Q}{c\Delta T}$
Result:
$m=\frac{ 2.0\times10^{6}J}{4186\,J/(kg\,^{\circ}C)\times80^{\circ}C}=6.0\,kg$