We can use distribution property to multiply the polynomial by the monomial.
$$ \begin{aligned} \left( 6x^2-3x \right) \cdot \color{orangered}{3x} &= 6x^2 \cdot \color{orangered}{3x} + \left( -3x\right) \cdot \color{orangered}{3x} = \\ &= 18x^3-9x^2 \end{aligned} $$