2p^2 + 3p - 4
- -2p^2 - 3p + 4
-------------------------
Change the subtraction to an addition by changing the sign of every term of the second polynomial.
2p^2 + 3p - 4
+ 2p^2 + 3p - 4
-------------------------
Now add like terms.
2p^2 + 3p - 4
+ 2p^2 + 3p - 4
-------------------------
4p^2 + 6p - 8
