First things first,definitions! Let X be the random variable for the number of shrubs per acre of jojoba.

In my opinion, X should follow a Poisson distribution, as there is theoretically no limit to the number of shrubs we can find, instead of a Binomial model as seen in your calculations. Hence, [tex] X \sim P_0(2) [/tex]
With this, you should be able to proceed with your working for (a).

As for part (b), I think we should pay some attention to the word "next". This probably means that there is some restriction on the probability. I think the question means that the agronomist should find 5 plants in the first area of land... but which area (1, 2, or 3 acres) should we use?