Spread of disease is density dependent because it is more severe the greater the population density as the disease would spread more easily. If there is low population density it would spread slower.
I'm not exactly sure what specifically you are asking for in relationship to balancing equations, but I have some examples with working out so you can examine them and try to go through the steps with them.
Hope this helps! <3
Answer:
Rhodopsin is the photopigment in rods.