You may have incorrectly distributed the 5 on the left side to the second x, and you may have forgotten to distribute the 3 on the right side of the equation. This would give you the incorrect equation $\displaystyle{5}{x}-{15}+{5}{x}={6}{x}-{5}\usepackage{cmbright}\Huge\$ . The correct distributions would give you $\displaystyle{5}{x}-{15}+{x}={6}{x}-{15}\usepackage{cmbright}\Huge\$ .