please answer both a and basap

A branching process is called binary fission if the offspring probability distribution has non-zero probabilities only for 0 or 2 offspring. Such a process starts at generation 0, with a single individual, and the probability of each individual producing 0 offspring is p. (a) Find the mean and variance of the size of the population at generation n. (b) Carefully distinguishing between the cases where ultimate extinction is certain or not, find the probability of ultimate extinction in terms of p