If each digit can be any number from 0 to 9 how many different credit numbers are possible

There are 16 digits in a credit card number, you know,
So any number 0-9 can be those digits. There are 10 numbers 0-9, including 0, so you would do 10^16, since numbers can be repeated.

First subtract 10 from both side
y^2+6y-16=0
(y+8)(y-2)=0
-8 and 2

I’m pretty sure the answer is a