The inequality can be rewritten as: $\displaystyle-{3}\le{3}{z}-{9}\le{3}\usepackage{cmbright}\Huge\$

Add 9 to all parts of the inequality: $\displaystyle{6}\le{3}{z}\le{12}\usepackage{cmbright}\Huge\$

Divide all parts of the inequality by 3: $\displaystyle{2}\le{z}\le{4}\usepackage{cmbright}\Huge\$