You may have correctly added 10 to both sides to get $\displaystyle{\left|{3}{y}+{7}\right|}={5}\usepackage{cmbright}\Huge\$ , but then solved only $\displaystyle{3}{y}-{7}={5}\usepackage{cmbright}\Huge\$ . If $\displaystyle{\left|{X}\right|}={5}\usepackage{cmbright}\Huge\$ , then $\displaystyle{X}={5}\usepackage{cmbright}\Huge\$ or $\displaystyle{X}=-{5}\usepackage{cmbright}\Huge\$ . In this problem, X is replaced by $\displaystyle{3}{y}-{7}\usepackage{cmbright}\Huge\$ . So you should be solving two equations, $\displaystyle{3}{y}-{7}={5}\usepackage{cmbright}\Huge\$ and $\displaystyle{3}{y}-{7}=-{5}\usepackage{cmbright}\Huge\$ , but you forgot one of them.