What are the explicit equation and domain for a geometric sequence with the first term of 4 and a second term of -8

A1 = ar^(n - 1)
a1 = 4
n = 1
r = doesn't matter yet
a = ???

4 = ar^(n - 1)
4 = ar^0
4 = a because anything to the zero is 1.

a2 = - 8
a = 4
r = ??
n = 2

- 8 = 4 r^(2 - 1)
- 8 / 4 = r^1
-2 = r

an = 4* (-2)^(n - 1) <<<<< explicit expression

Answer an = 4*(-2) ^ (n - 1)

108