You may have forgotten to distribute the 7 to the x on the right side of the equation. The correct simplification is $\displaystyle{5}{x}+{20}-{7}{x}={21}+{7}{x}-{3}{x}\usepackage{cmbright}\Huge\$ . This will simplify further to $\displaystyle-{2}{x}+{20}={21}+{4}{x}\usepackage{cmbright}\Huge\$ . This equation has the solution $\displaystyle{x}=-\frac{{1}}{{6}}\usepackage{cmbright}\Huge\$ . The correct answer is $\displaystyle{5}{x}+{20}-{x}={7}{\left({3}+{x}\right)}-{3}{x}\usepackage{cmbright}\Huge\$ .