Subtract 7 from both sides: $\displaystyle{4}{x}={9}+{x}\usepackage{cmbright}\Huge\$

Subtract x from both sides: $\displaystyle{3}{x}={9}\usepackage{cmbright}\Huge\$

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