Use the distributive property on the left side to simplify:

$\displaystyle{12}{x}-{4}+{9}-{2}{x}={2}{x}+{5}+{8}{x}\usepackage{cmbright}\Huge\$

Combine like terms on both sides: $\displaystyle{10}{x}+{5}={10}{x}+{5}\usepackage{cmbright}\Huge\$

Subtract $\displaystyle{10}{x}\usepackage{cmbright}\Huge\$ from both sides: $\displaystyle{5}={5}\usepackage{cmbright}\Huge\$

This is true no matter what x is, so the solution is all real numbers.