Need help on this exercise

Exercise 8. We will build a compartment model for the flow of nitrogen from soil fungi to the plant roots that host them. The compartment is the soil fungi, and the flow we are is nitrogen absorbed by the fungi and then released to the roots. Let x(t) be the nitrogen in the fungi, measured in grams. Assume that in one unit of time the fungi absorb 1 g of nitrogen and release 20% of whatever nitrogen they have, to the plant. (a) Derive a differential equation for x(t), explaining your reasoning as best as you can. (b) By solving your differential equation, show that no matter what the starting value of x(0) is, x(t) converges to a limit (which you should calculate) as t oo