You may have thought that the notation $\displaystyle{\left({f{\cdot}}{g}\right)}{\left({x}\right)}\usepackage{cmbright}\Huge\$ meant that you had to multiply the product by a factor of x, but it does not. Start with:

$\displaystyle{\left({f{\cdot}}{g}\right)}{\left({x}\right)}={f{{\left({x}\right)}}}\cdot{g{{\left({x}\right)}}}={8}{{x}}^{{4}}\cdot{2}{{x}}^{{2}}\usepackage{cmbright}\Huge\$

Multiply the coefficients and add the exponents.