You may have incorrectly distributed the 5 on the left side to the second x. This would give you the incorrect equation $\displaystyle{5}{x}-{15}+{5}{x}={6}{x}-{15}\usepackage{cmbright}\Huge\$ . The left side should be $\displaystyle{5}{x}-{15}+{x}\usepackage{cmbright}\Huge\$ .