You incorrectly tried to add $\displaystyle-{10}{{g}}^{{2}}\usepackage{cmbright}\Huge\$ and $\displaystyle-{2}{g{}}\usepackage{cmbright}\Huge\$ , but these are unlike terms and cannot be combined. First regroup to match like terms, and then combine them. The correct answer is $\displaystyle-{10}{{g}}^{{2}}+{12}{g{+}}{4}\usepackage{cmbright}\Huge\$ .