Answer
$(-\infty,\infty)$.
Work Step by Step
Step 1. Given $f(x)=3x^2-4$ and $g(x)=x^2-3x-4$, we can find the domain for $f$ is $(-\infty,\infty)$ and the domain for $g$ is $(-\infty,\infty)$.
Step 2. Thus the domain for $(fg)(x)$ is $(-\infty,\infty)$.
