## Elementary and Intermediate Algebra: Concepts & Applications (6th Edition)

$$x^2+6x-9$$
We know the following: $$(f-g)(x)=f(x)-g(x)$$ Thus, plugging in $f(x)$ and $g(x)$ and combining like terms, we find: $$x^2+3x-\left(9-3x\right)\\ x^2+3x-9+3x\\ x^2+6x-9$$