It appears that you subtracted in the wrong order. Start with the following:

$\displaystyle{\left({f{-}}{g}\right)}{\left({x}\right)}={f{{\left({x}\right)}}}-{g{{\left({x}\right)}}}={9}{x}+{6}-{3}{x}\usepackage{cmbright}\Huge\$

Rearrange and combine like terms.