12. (2v^3 - v + 8) + (-v^3 + v - 3)

14. (4h^3 + 3h + 1) - (-5h^3 + 6h - 2)

12. On addition, you can just combine like terms.
2v^3+(-v^3)=v^3
-v+v cancels each other out
8+(-3)=5
So you have v^3+5

14 On subtraction, you have to remember to distribute the negative sign so after you do that you have:
4h^3+3h+1+5h^3-6h+2
Then you can combine like terms
4h^3+5h^3=9h^3
3h-6h=-3h
1+2=3
So you end up with:
9h^3-3h+3

Hope that helps and feel free to ask any questions.