We use the exponential growth formula here.

where is the population at time , is the initial population, is the rate of growth, and is time.

(a)

we are told that after 4 hours, the population triples. so if we start with at , we will have when . So let's plug those values into our equation.

So we have,

we are also told that when , the population is 1000000

so we have:

now solve for

once you find , you will have the equation that models this growth, part (b) won't be difficult after that. So find and try part (b) and we'll see how far you get

EDIT: This is my 33th post!!!