Combine like terms: $\displaystyle-{12}{x}+{12}=-{15}{x}+{13}\usepackage{cmbright}\Huge\$

Subtract 12 from both sides: $\displaystyle-{12}{x}=-{15}{x}+{1}\usepackage{cmbright}\Huge\$

Add $\displaystyle{15}{x}\usepackage{cmbright}\Huge\$ to both sides: $\displaystyle{3}{x}={1}\usepackage{cmbright}\Huge\$

Divide both sides by 3: $\displaystyle{x}=\frac{{1}}{{3}}\usepackage{cmbright}\Huge\$