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

$\displaystyle{6}{c}-{3}={15}\usepackage{cmbright}\Huge\$ or $\displaystyle{6}{c}-{3}=-{15}\usepackage{cmbright}\Huge\$

Add 3 to both sides of both equations: $\displaystyle{6}{c}={18}\usepackage{cmbright}\Huge\$ or $\displaystyle{6}{c}=-{12}\usepackage{cmbright}\Huge\$

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