Aparticular type of cell doubles in number every hour. which function can be used to find the number of cells present at the end of h hours if there are initially 4 of these cells

The function would be 
.

This is in the general form 
, where a is the initial value, b is the rate it increases by, and x is the amount of time.  For our function, the initial value is 4 and the rate it increases by is 2.