You multiplied the expression in the function by $\displaystyle{\left({x}-{3}\right)}\usepackage{cmbright}\Huge\$ , instead of substituting $\displaystyle{\left({x}-{3}\right)}\usepackage{cmbright}\Huge\$ for x. Your first step should be to replace x by $\displaystyle{\left({x}-{3}\right)}\usepackage{cmbright}\Huge\$ : $\displaystyle{f{{\left({x}-{3}\right)}}}={6}{\left({x}-{3}\right)}+{2}\usepackage{cmbright}\Huge\$ . Then simplify this expression.