Math Question:?

[jeremy.smyth22]

Consider the following model for a drug prescription:
a_(n+1) = a_(n) - ka_(n) + b,
where a_(n) is the amount of a drug (in mg, say) in the bloodstream after administration of n dosages at regular intervals (hourly, say), k is the rate at which the drug in eliminated from the body and b is the amount added when a new dose is taken.

- Perform a cobwebbing analysis for this model. What happens to the amount of drug in the bloodstream in the long run? How does the result depend on the model parameters?

- How should b be chosen to ensure that the drug is effective, but not toxic?

Note: The underscores (or rather '_') mean subscript, in this case.

Please help - I have been working on this question for many days now, and have got almost nowhere! If someone could help me (so I can understand how to work this question out), it would be much appreciated!
Thanks.