Question

A system conducts 255 cal of heat to the surroundings while delivering 428 calories of work. What is the change in the internal energy of the system in (cal)?

I know the answer is -683 calories already, I just want to know why I know when the system is doing work on the surrounding we get a negative value hence the (-255 calories), but why is the 428 calories a negative value as well when trying to solve for the answer?