Geometric series: sum to infinity

No that is wrong. When did it say n=0. I'm pretty sure it starts at n=1 and goes on.

Again, your wrong it works fine without replacing n with n-1.

Can we please go back and actually discuss if the question is right or not.

"Sum the series 1 + 4x + 7x² + 10x³ + ... + (3N - 2)x^N."

so the general term is (3N - 2)x^N

Putting N = 0 gives -2
Putting N = 1 gives (3 - 2)x^1 = x
Putting N = 2 gives (6 - 2)x^2 = 4x^2
Putting N = 2 gives (9 - 2)x^3 = 7x^3

So the series has to be (3n - 2)x^(n-1)