You may have correctly set up the two equations $\displaystyle-{4}{d}+{7}={21}\usepackage{cmbright}\Huge\$ and $\displaystyle-{4}{d}+{7}=-{21}\usepackage{cmbright}\Huge\$ , but perhaps forgot that a positive number divided by a negative number gives a negative quotient. The correct answer is $\displaystyle{d}=-\frac{{7}}{{2}}\usepackage{cmbright}\Huge\$ or $\displaystyle{d}={7}\usepackage{cmbright}\Huge\$ .