Add 18 to all parts: $\displaystyle-{9}+{18}\le{3}{p}-{18}+{18}\le{3}+{18}\usepackage{cmbright}\Huge\$

Simplify: $\displaystyle{9}\le{3}{p}\le{21}\usepackage{cmbright}\Huge\$

Divide all parts by 3: $\displaystyle{3}\le{p}\le{7}\usepackage{cmbright}\Huge\$