How do you multiply 3x^2(2x^2+20x+12)?

To multiply this expression you multiply $\textcolor{red}{3 {x}^{2}}$ by each term within parenthesis:
$\left(\textcolor{red}{3 {x}^{2}} \times 2 {x}^{2}\right) + \left(\textcolor{red}{3 {x}^{2}} \times 20 x\right) + \left(\textcolor{red}{3 {x}^{2}} \times 12\right) \to$
$6 {x}^{4} + 60 {x}^{3} + 36 {x}^{2}$