It seems that you added 3 instead of multiplying both sides of the equation by 3. Adding 3 to $\displaystyle\frac{{x}}{{3}}\usepackage{cmbright}\Huge\$ 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\$ .