In order to multiply monomials, multiply the coefficients and add the exponents.

$\displaystyle{15}{n}\cdot{\left(-{4}{{n}}^{{3}}\right)}={15}\cdot{\left(-{4}\right)}\cdot{{n}}^{{1}}\cdot{{n}}^{{3}}=-{60}{{n}}^{{{1}+{3}}}=-{60}{{n}}^{{4}}\usepackage{cmbright}\Huge\$