1st term = 4, 13th term = 16384

In a geometric progression (GP), every term is got by multiplying the previous
term by a common ratio (r).

So if a is the 1st term, r is the common ratio
1st term = a
2nd term = a*r
3rd term = a*r*r = a*r^2
In general, nth term will be 

Here we need to find what is the common ratio.

a = 4, 13th term = 




 (you can find it using a calculator, or by factorizing 4096)

So, a = 4 and r = 2 in this series.

Formula for nth term = 

Hope you got it :)