We can use distribution property to multiply the polynomial by the monomial.
$$ \begin{aligned} \color{blue}{3x} \cdot \left( 4x^2+x-4 \right) &= \color{blue}{3x} \cdot 4x^2 + \color{blue}{3x} \cdot x + \color{blue}{3x} \cdot \left( -4\right) = \\ &= 12x^3+3x^2-12x \end{aligned} $$