how would you approach this question? I need to know step by step

Crystals are dissolved in a liquid solution non saturated; it is proposed that the diffusivity DAB from Fick' equation Naz = XA (NA + N82) - CDAB (dxx/dz) It is assumed that the system is at steady state, there is not chemical reaction and that the volumetric flow of the ascendent solution is equal to the crystal dissolved, in a way that the following expression is valid [(MA Naz) + (Mo Na)]/p=-(MA Naz)/ Ps Where ps is the crystal density and pis de solution density Obtain: a. Expression to evaluate based on experiments the diffusivity Das considering that this substance concentration does not change significantly within the system. YAZI P XA, 2 YAoj 20