To simplify an expression, reorder the addends and combine like terms.

$\displaystyle-{14}{c}{d}-{2}{{c}}^{{2}}{{d}}^{{2}}+{15}{c}{d}+{8}{{c}}^{{2}}{{d}}^{{2}}+{15}{c}{d}=-{14}{c}{d}+{15}{c}{d}+{15}{c}{d}-{2}{{c}}^{{2}}{{d}}^{{2}}+{8}{{c}}^{{2}}{{d}}^{{2}}\usepackage{cmbright}\Huge\$

$\displaystyle={\left(-{14}+{15}+{15}\right)}{c}{d}+{\left(-{2}+{8}\right)}{{c}}^{{2}}{{d}}^{{2}}={16}{c}{d}+{6}{{c}}^{{2}}{{d}}^{{2}}\usepackage{cmbright}\Huge\$