You may have correctly found the value of $\displaystyle{4}{x}\usepackage{cmbright}\Huge\$ instead of the value of $\displaystyle{10}-{4}{x}\usepackage{cmbright}\Huge\$ . The correct answer is that $\displaystyle{x}={1}\usepackage{cmbright}\Huge\$ will give you the larger value of 6.