If the power is divisible by 2 and not 4, it is -1.
If the power is divisible by 4, it is 1.
If it satisfys none of those conditions, it is i or -i.
(basically, it is a pattern of i, -1, -i, and 1)
So, let's see what i^11 is. By running down the list, it satisfys only -i. Which means, i^11 is -i.
i^16 is divisible by 4, so it is 1.
Keep on going like this until you have the answer!