The original equation can be rewritten as two equations without absolute value:

$\displaystyle-{4}{d}+{7}={21}\usepackage{cmbright}\Huge\$ or $\displaystyle-{4}{d}+{7}=-{21}\usepackage{cmbright}\Huge\$

Subtract 7 from both sides of both equations: $\displaystyle-{4}{d}={14}\usepackage{cmbright}\Huge\$ or $\displaystyle-{4}{d}=-{28}\usepackage{cmbright}\Huge\$

Divide both sides of both equations by $\displaystyle-{4}\usepackage{cmbright}\Huge\$ : $\displaystyle{d}=-\frac{{7}}{{2}}\usepackage{cmbright}\Huge\$ or $\displaystyle{d}={7}\usepackage{cmbright}\Huge\$