Substitute the expression $\displaystyle{\left({x}-{3}\right)}\usepackage{cmbright}\Huge\$ into the function for x: $\displaystyle{f{{\left({x}-{3}\right)}}}={6}{\left({x}-{3}\right)}+{2}\usepackage{cmbright}\Huge\$

Use the distributive property on the right side: $\displaystyle{f{{\left({x}-{3}\right)}}}={6}{x}-{18}+{2}\usepackage{cmbright}\Huge\$

Combine like terms to simplify: $\displaystyle{f{{\left({x}-{3}\right)}}}={6}{x}-{16}\usepackage{cmbright}\Huge\$