We can use distribution property to multiply the polynomial by the monomial.
$$ \begin{aligned} \color{blue}{5x} \cdot \left( 6x^2+8x+6 \right) &= \color{blue}{5x} \cdot 6x^2 + \color{blue}{5x} \cdot 8x + \color{blue}{5x} \cdot 6 = \\ &= 30x^3+40x^2+30x \end{aligned} $$