Answer
$$ -4x^2 + 6x+15$$
Work Step by Step
Like terms must (1) be raised to the same power and (2) consist of the same variable (i.e. x and y are not like terms). It is also important to note that numbers are all like terms with each other when not multiplied by anything. Thus, combining like terms, we obtain:
$$=3x+6 -4x^2 +3x +9 \\ = -4x^2 + 6x+15$$
