$y = f(x) = \frac{3x^2}{x^2 - 9} = \frac{3x^2}{(x-3)(x+3)}$

Vertical asymptotes where denominator $= 0$: $x = -3 \text{ and } x = 3$

Horizontal asymptote (degree of numerator = degree of denominator): $y = \frac{3}{1} = 3$

Asymptotes: $x = -3$, $x = 3$, and $y = 3$.