I came across a GP questions as follows-

The difference between 5th term and 3rd term of GP is 72. If the 1st term is 1. Find the common ratio.

I had solved it. Please correct me. Where am I wrong?

a= 1 as its the 1st term. ar^4 - ar^2 = 72. This implies r^4- r^2 = 72. From here I can solve r. But the answers are 2, 3, 4 or 5.