Use the distributive property first, and then combine like terms on both sides. The equation will simplify to $\displaystyle{4}{x}+{21}={21}+{4}{x}\usepackage{cmbright}\Huge\$ . Subtract $\displaystyle{4}{x}\usepackage{cmbright}\Huge\$ from both sides to get $\displaystyle{21}={21}\usepackage{cmbright}\Huge\$ . Since this is true no matter what value x is, it means that every value of x is a solution. That is, the solution is all real numbers. The correct answer is $\displaystyle{5}{x}+{20}-{x}={7}{\left({3}+{x}\right)}-{3}{x}\usepackage{cmbright}\Huge\$ .