You may have subtracted $\displaystyle{75}-{54}\usepackage{cmbright}\Huge\$ . When a number is next to a variable, like $\displaystyle{7}{a}\usepackage{cmbright}\Huge\$ or like $\displaystyle{5}{b}\usepackage{cmbright}\Huge\$ , it means that you have to multiply. So you should multiply 7 and 5, multiply 5 and 4, and then subtract.