Substituting $\displaystyle\frac{{{P}-{s}{q}}}{{-{{2}}}}\usepackage{cmbright}\Huge\$ for q into $\displaystyle{q}=-{750}{s}+{7000}\usepackage{cmbright}\Huge\$ will not create a quadratic equation. Could she substitute a quantity into the equation $\displaystyle{P}={s}{q}-{2}{q}\usepackage{cmbright}\Huge\$ instead?