Answer
(a) $119J$
(b) $35J$
(c) $0J$
Work Step by Step
(a) According to first law of thermodynamics
$\Delta U=Q-W$
We plug in the known values to obtain:
$\Delta U=77-(-42)$
$\Delta U=119J$
(b) As we know that
$\Delta U=Q-W$
We plug in the known values to obtain:
$\Delta U=77-42=35J$
(c) As $\Delta U=Q-W$
This can be rearranged as:
$Q=W+\Delta U$
We plug in the known values to obtain:
$Q=-120+120=0J$
