We can use distribution property to multiply the polynomial by the monomial.
$$ \begin{aligned} \left( -x^2+5x \right) \cdot \color{orangered}{3x^2} &= \left( -x^2\right) \cdot \color{orangered}{3x^2} + 5x \cdot \color{orangered}{3x^2} = \\ &= -3x^4+15x^3 \end{aligned} $$