Find the limits as $x \to \infty$ and as $x \to -\infty$. (If the limit is infinite, enter '$\infty$' or '$-\infty$', as appropriate. If the limit does not otherwise exist, enter DNE.)
$\lim_{x\to\infty} f(x) = \text{________}$
$f(x) = 3x^3 - x^4$
$\lim_{x\to-\infty} f(x) = \text{________}$
Use this information, together with intercepts, to give a rough sketch of the graph as in this example.