Answer
$\frac{x-3}{x-6}$, $x\ne\pm6$
Work Step by Step
Step 1. The domain requirement of the rational expression is that $x^2-36\ne0$, which gives $x\ne\pm6$
Step 2. Factor the numerator and the denominator completely. We have $\frac{x^2+3x-18}{x^2-36}=\frac{(x+6)(x-3)}{(x+6)(x-6)}$
Step 3. Cancel the common factors from the above expression. We get the result as $\frac{x-3}{x-6}$ with $x\ne\pm6$
