Add $\displaystyle{2}{s}\usepackage{cmbright}\Huge\$ to both sides: $\displaystyle{q}+{2}{s}={3}{r}\usepackage{cmbright}\Huge\$

Divide both sides by 3: $\displaystyle\frac{{{q}+{2}{s}}}{{3}}=\frac{{{3}{r}}}{{3}}\usepackage{cmbright}\Huge\$

Simplify: $\displaystyle\frac{{{q}+{2}{s}}}{{3}}={r}\usepackage{cmbright}\Huge\$