You mixed up the directions of the symbols when rewriting the solution. The solution is $\displaystyle{t}\le-{2}\usepackage{cmbright}\Huge\$ and $\displaystyle{t}\ge-{7}\usepackage{cmbright}\Huge\$ . This means that the values of t are at least $\displaystyle-{7}\usepackage{cmbright}\Huge\$ and at most $\displaystyle-{2}\usepackage{cmbright}\Huge\$ . The compound inequality given does not say this. The correct solution is $\displaystyle-{7}\le{t}\le-{2}\usepackage{cmbright}\Huge\$ .