You may have thought that the notation $\displaystyle{\left({f{\cdot}}{g}\right)}{\left({x}\right)}\usepackage{cmbright}\Huge\$ meant that you should multiply the product of f and g by x, but it does not. The correct interpretation of this notation is:

$\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.