How many different arrangements of the letters in the word "eyes" are there?

It's a combination,

there are 2 "e" s, 1"y", and 1"s"

there are 4 letters in total

if there weren't repeats you could write 4! (4*3*2*1)

but since the "e"s repeat you have to write it

4!/2! which equals 4*3*2*1/2*1 so 12