You may have substituted incorrectly and computed $\displaystyle{8}{\left({6}\right)}-{3}{\left({7}\right)}\usepackage{cmbright}\Huge\$ . You need to compute $\displaystyle{8}{\left({7}\right)}-{3}{\left({6}\right)}\usepackage{cmbright}\Huge\$ . The correct answer is 38.