To multiply a polynomial by a monomial, be sure to distribute the monomial to each term in the polynomial.

$\displaystyle{12}{{x}}^{{3}}{\left({3}{{x}}^{{5}}-{2}{{x}}^{{3}}-{5}{x}\right)}={12}{{x}}^{{3}}{\left({3}{{x}}^{{5}}\right)}+{12}{{x}}^{{3}}{\left(-{2}{{x}}^{{3}}\right)}+{12}{{x}}^{{3}}{\left(-{5}{x}\right)}={36}{{x}}^{{8}}-{24}{{x}}^{{6}}-{60}{{x}}^{{4}}\usepackage{cmbright}\Huge\$