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

$\displaystyle{\left({f{-}}{g}\right)}{\left({x}\right)}={f{{\left({x}\right)}}}-{g{{\left({x}\right)}}}={3}{x}+{4}-{9}\usepackage{cmbright}\Huge\$

Combine like terms.