Use a test point in each region. Point $\displaystyle{\left({1},{8}\right)}\usepackage{cmbright}\Huge\$ makes $\displaystyle{b}+{t}\ge{8}\usepackage{cmbright}\Huge\$ false, so region 1 is not shaded. Point $\displaystyle{\left({3},{8}\right)}\usepackage{cmbright}\Huge\$ makes both inequalities true, so region 2 is shaded. Point $\displaystyle{\left({6},{1}\right)}\usepackage{cmbright}\Huge\$ makes $\displaystyle{b}+{t}\ge{8}\usepackage{cmbright}\Huge\$ false, so region 3 is not shaded. Point (1, 1) makes both inequalities false, so region 4 is not shaded.