You may have correctly used the distributive property in the first step and added 7 to both sides to get $\displaystyle{7}{x}={3}{x}+{20}\usepackage{cmbright}\Huge\$ . However, here you may have tried to add $\displaystyle{3}{x}\usepackage{cmbright}\Huge\$ to isolate the variable term. You need to subtract $\displaystyle{3}{x}\usepackage{cmbright}\Huge\$ from both sides to do this.