Answer
$$\frac{x-3}{x+5}$$
Work Step by Step
Recall, to simplify rational expressions, we look for common factors in the numerator and the denominator. Doing this, we find:
$$ \frac{3x^2-9x}{3x^2+15x}\\ \frac{3x\left(x-3\right)}{3x\left(x+5\right)}\\ \frac{x-3}{x+5}$$
