You did not reverse the inequality symbol when dividing both sides of each inequality by a negative number, and so you thought the solution was $\displaystyle{b}\ge{4}\usepackage{cmbright}\Huge\$ or $\displaystyle{b}\le{9}\usepackage{cmbright}\Huge\$ , which would include all real numbers. You need to remember to reverse the inequality symbol whenever you multiply or divide both sides of an inequality by a negative number. The correct answer is $\displaystyle{b}\le{4}\usepackage{cmbright}\Huge\$ or $\displaystyle{b}\ge{9}\usepackage{cmbright}\Huge\$ .