I read the numbers placed after the variables as powers.
If I write the expression substituting the variable values given you should be able to see how the calculation is done.
The first thing to do is to combine similar terms:
-a^2-b^3 because the b^3 terms combine and the c^2 terms cancel out.
(^ means “to the power of”.) Only two terms left: -3*3-2*2*2=-9-8=-17.
I hope you and your brother can understand the working.