Use the distributive property on the left side to simplify:

$\displaystyle{12}-{4}{x}+{6}{x}={3}{x}+{12}-{x}\usepackage{cmbright}\Huge\$

Combine like terms on both sides: $\displaystyle{12}+{2}{x}={2}{x}+{12}\usepackage{cmbright}\Huge\$

Subtract $\displaystyle{2}{x}\usepackage{cmbright}\Huge\$ from both sides: $\displaystyle{12}={12}\usepackage{cmbright}\Huge\$

This means that the solution is all real numbers.