Correct. Substitute $\displaystyle-{2}\usepackage{cmbright}\Huge\$ for x in the function: $\displaystyle{a}{\left(-{2}\right)}={3}{{\left(-{2}\right)}}^{{2}}-{6}\usepackage{cmbright}\Huge\$

Simplify the expression: $\displaystyle{a}{\left(-{2}\right)}={3}{\left({4}\right)}-{6}={12}-{6}={6}\usepackage{cmbright}\Huge\$