Answer
$-3x^2-21x-2$
Work Step by Step
We simplify by removing the parentheses and then add the like terms as follows:
$$
(-x^2 + 6x - 8) + (10x^2 - 13x + 3) +
(-12x^2 - 14x + 3)\\
=-x^2 + 6x - 8 + 10x^2 - 13x + 3 -12x^2 - 14x + 3\\
=-3x^2-21x-2
.$$
