As the number of cells doubles every 2.5 monts, the number of cells can be written as
N = A * 2^m
where
A is the starting amount
N is the number of cells at any given time
m is the given months divided by 2.5
Thus,
1) For 2 years,
m = 2 * 12/2.5 = 9.6
N = 2^9.6
N = 776
There would be 776 cells after 2 years
2) For 3 years,
m = 3 * 12/2.5 = 14.4
N = 2^14.4
N = 21619
There would be 21619 cells after 3 years