PLEASE HELP What are the explicit equation and domain for a geometric sequence with a first term of 2 and a second term of −8?

an = 2(−8)n − 1; all integers where n ≥ 1
an = 2(−8)n − 1; all integers where n ≥ 0
an = 2(−4)n − 1; all integers where n ≥ 0
an = 2(−4)n − 1; all integers where n ≥ 1