It appears that you did not distribute the factor $\displaystyle{3}{x}\usepackage{cmbright}\Huge\$ to all terms in the second function. Start with:

$\displaystyle{\left({f{\cdot}}{g}\right)}{\left({x}\right)}={f{{\left({x}\right)}}}\cdot{g{{\left({x}\right)}}}={\left({3}{x}\right)}{\left({{x}}^{{2}}-{5}{x}-{9}\right)}\usepackage{cmbright}\Huge\$

Use the distributive property.