So, a scientist is growing a colony of bacteria. After 2 hours, there are 40 bacteria. After 3 hours, there are 120 bacteria. If the population of bacteria grows exponentially, then how many bacteria will there be after 4 hours?

We have (2, 40), (3, 120), and we need to find (4, ?). I've done the following:

, and

2)

3)

4)

That's where I am. In most of the examples I've seen, in step 3, the equation is usually being multiplied by something simple, like , but for this one, I guess I have to divide , which in step 4, I am confused.