You may have tried to solve by changing the signs to $\displaystyle{4}{d}-{7}={21}\usepackage{cmbright}\Huge\$ and just solving this equation. However, if $\displaystyle{\left|{X}\right|}={21}\usepackage{cmbright}\Huge\$ , then $\displaystyle{X}={21}\usepackage{cmbright}\Huge\$ or $\displaystyle{X}=-{21}\usepackage{cmbright}\Huge\$ . In this problem, X is replaced by $\displaystyle-{4}{d}+{7}\usepackage{cmbright}\Huge\$ . So you should be solving two equations, $\displaystyle-{4}{d}+{7}={21}\usepackage{cmbright}\Huge\$ and $\displaystyle-{4}{d}+{7}=-{21}\usepackage{cmbright}\Huge\$ . The correct answer is $\displaystyle{d}=-\frac{{7}}{{2}}\usepackage{cmbright}\Huge\$ or $\displaystyle{d}={7}\usepackage{cmbright}\Huge\$ .