Use the distributive property on the left side:

$\displaystyle{24}{r}+{12}\ge{20}{r}-{8}\usepackage{cmbright}\Huge\$

Subtract 12 from both sides: $\displaystyle{24}{r}\ge{20}{r}-{20}\usepackage{cmbright}\Huge\$

Subtract 20r from both sides: $\displaystyle{4}{r}\ge-{20}\usepackage{cmbright}\Huge\$

Divide both sides by 4: $\displaystyle{r}\ge-{5}\usepackage{cmbright}\Huge\$