When multiplying a monomial by a polynomial, use the distributive property.

$\displaystyle{4}{a}{b}{\left({7}{a}-{2}{b}\right)}={4}{a}{b}{\left({7}{a}\right)}-{4}{a}{b}{\left({2}{b}\right)}={28}{{a}}^{{2}}{b}-{8}{a}{{b}}^{{2}}\usepackage{cmbright}\Huge\$