Geometric calculations problem

Question

Q: Find a_21 if a_1=9 and d=-7/2

I used the geometric sequence formula, and I got about 6.8486 (rounding to four decimal places). Not sure if this is correct, any help is great! Thanks.

Answer 1

a_n = a₁(r)^n-1

a₂₁ = 9(-7/2)²⁰

Can you calculate and anwer?