You may have combined unlike terms: $\displaystyle{4}{x}+{3}{x}\usepackage{cmbright}\Huge\$ would simplify to $\displaystyle{7}{x}\usepackage{cmbright}\Huge\$ , but $\displaystyle{4}{x}+{3}\usepackage{cmbright}\Huge\$ would not. Likewise, $\displaystyle{5}+{7}={12}\usepackage{cmbright}\Huge\$ , but $\displaystyle{5}{x}+{7}\usepackage{cmbright}\Huge\$ does not simplify to 12.