$${\mathtt{3}}{\mathtt{\,\times\,}}{\mathtt{x}}{\mathtt{\,\small\textbf+\,}}{{\mathtt{x}}}^{{\mathtt{2}}}$$
∫3 x^2 dx =
3 [x^4 / 4 ] + C =
(3/4)x^4 + C
(If you differentiate this, you will get the function under the integrand, i.e., 3x^2 )
You can't. It is as simplified as possible.