We can get rid of the parenthesees since there is only addition between them.
= 3x2 + 9x - 1 + -5x2 + 6x - 10
Now we can rearrange the terms so like terms are together.
= 3x2 - 5x2 + 9x + 6x - 1 - 10
Combine like terms. Just like 3 apples - 5 apples = -2 apples ....so does 3x2 - 5x2 = -2x2
= -2x2 + 15x - 11