The original equation can be rewritten as two equations without absolute value:

$\displaystyle{3}{c}+{6}={12}\usepackage{cmbright}\Huge\$ or $\displaystyle{3}{c}+{6}=-{12}\usepackage{cmbright}\Huge\$

Subtract 6 from both sides of both equations: $\displaystyle{3}{c}={6}\usepackage{cmbright}\Huge\$ or $\displaystyle{3}{c}=-{18}\usepackage{cmbright}\Huge\$

Divide both sides of both equations by 3: $\displaystyle{c}={2}\usepackage{cmbright}\Huge\$ or $\displaystyle{c}=-{6}\usepackage{cmbright}\Huge\$