It seems that you divided by 3 instead of multiplying both sides of the equation by 3. Dividing $\displaystyle\frac{{x}}{{3}}\usepackage{cmbright}\Huge\$ by 3 will not leave you with x, and you must do the same thing to both sides. The correct answer is $\displaystyle{x}={27}\usepackage{cmbright}\Huge\$ .