There's this problem I have to do for calc, and I'm having a hard time getting it started.

The number of bacteria in a culture is increasing according to the law of exponential growth. There are 125 bacteria in the culture after 2 hours and 350 bacteria after 4 hours.

A) Find the initial population

We've only done problems where the initial population has been given to us. Could someone explain how to find it?

Thanks, Jeff