\displaystyle \boxed{y_{q_i}=\frac{F_iq_{F_i}-S_iq_{S_i}}{\left({1-q_{S_i}}\right) S_i-\left({1-q_{F_i}}\right) F_i}x_{q_i}-\frac{F_ix_{F_i
\displaystyle\boxed{y_{q_i}=\frac{F_iq_{F_i}-S_iq_{S_i}}{\left({1-q_{S_i}}\right)S_i-\left({1-q_{F_i}}\right)F_i}x_{q_i}-\frac{F_ix_{F_i}-S_ix_{S_i}}{\left({1-q_{S_i}}\right)S_i-\left({1-q_{F_i}}\right)F_i}} 