So assuming that this is a closed system. The internal energy is 93J-42 J= 51J
Putting it in perspective:
In laymen’s term, I ate 93J of food. I used 42 J to do my homework. What is the net change in my energy? So let’s assume you initially had an energy of E. You gained 93J of energy after eating. So, E+93J. Now you did your homework, which requires 42 J of energy to do. So currently the energy inside your system is E+93J before you do your homework. After you did your homework you spent 42 J, so it will be E+93J-42J, or E+51J