Volume is Vr$,$ influent flow rate is Q$,$ and substrate concentration is So$.$ The influent also contains active biomass at the concentration of Xa$,$o$.$ In the reactor, the active biomass is Xa$,$ and the substrate concentration is S at steady state. The following parameters are known: b$,$ Y$.$ The treatment process can be represented by the matrix provided. Assuming that unit conversions aren't needed, derive an expression that can be used to calculate the active biomass concentration in the reactor $($Xa$)$ as a function of solid retention time $($theta$\_$x$,)$ hydraulic retention time $($theta$),$ So$,$ S$,$ Y$,$ and b$.$ The reactor is at steady state.