$\displaystyle{f{{\left({x}\right)}}}={7}{x}+{2}\usepackage{cmbright}\Huge\$

$\displaystyle{g{{\left({x}\right)}}}={{x}}^{{2}}+{2}{x}\usepackage{cmbright}\Huge\$

$\displaystyle{\left({f{\cdot}}{g}\right)}{\left({x}\right)}={7}{{x}}^{{3}}+?{{x}}^{{2}}+{4}{x}\usepackage{cmbright}\Huge\$

The question mark stands for the number _____.