Answer
$$\frac{-3x+18}{\left(x-3\right)\left(x+4\right)}$$
Work Step by Step
Ensuring that we have like denominators and then attempting to simplify, we find:
$$\frac{3x}{\left(x-3\right)\left(x+4\right)}-\frac{6\left(x-3\right)}{\left(x+4\right)\left(x-3\right)}\\ \frac{3x-6\left(x-3\right)}{\left(x-3\right)\left(x+4\right)}\\ \frac{-3x+18}{\left(x-3\right)\left(x+4\right)}$$
