If the thermal energy of a system increases by 400 j , and 1100 j of heat was added to the system how much work did the system do?

The equation used here is ΔU=Q-W,
where, ΔU is the change in the system's internal energy, Q is the energy transferred to or from the system by  heat, and W is the energy transferred to or from the system by work.
 
Known:
ΔU=400J
Q=1100 J
W=?

Plug into the equation:
400=1100-W
Therefore W=700 J.

So the system did 700 J of work.