When isolating the variable, make sure that you perform the same operation on both sides of the equation. For example, if you add $\displaystyle{2}{s}\usepackage{cmbright}\Huge\$ to the right side, you must also add $\displaystyle{2}{s}\usepackage{cmbright}\Huge\$ to the left side. The correct answer is: $\displaystyle{r}=\frac{{{q}+{2}{s}}}{{3}}\usepackage{cmbright}\Huge\$