Let the arithmetic series be .

since the first terms of each sequence are the same, we know that the geometric sequence is of the form (the same )

this is for

hence, from the arithmetic sequence we have

from the geometric series we know that

now, know that you will be able to set up a system of equations to solve for , and hence answer the problem.

(i hope you know the formulas for an arithmetic sequence and geometric sequence to know how i got the expressions for and