\frac{d N_{2}}{d t} = 0 = - A_{21} N_{2} - B_{21} ρ_{f} N_{2} + B_{12} ρ_{f} N_{1}

${\frac{\mathrm{d}N_2}{\mathrm{d}t}= 0 = -A_{21} N_2-B_{21} \rho_f N_2+B_{12} \rho_f N_1}$

Error