Answer
$$ -9x^2+5x+11. $$
Work Step by Step
By adding the like $x^2$ terms, we have $$3x^2-9x^2-3x^2=-9x^2$$ and for the $x$ terms, we have $$6x+6x-7x=5x$$ and finally, for the constant terms, we get $$-8+15+4=11$$
Hence the final result is
$$ -9x^2+5x+11. $$
