i got some problems while i was doing exercises on exponential functions

and here it is. a table set

The question is to make some estimate of time taken for the population to double with a theoretical formula of the form Pt = Po × 2^at

where t is in years and Po and a are some particular values

I reckon that Po is the initial population, but which number should i use?

And how do I find the value of a?

I am really struggling with this topic and would like some help

any help would be highly appreciated

thank you