What is the length of a diagonal of a cube with a side length of 3 cm? (all of the answers are square rooted) 18,27,33,38

The three dimensional diagonal of a rectangular prism is:

d^2=x^2+y^2+z^2, and since this is a cube, x=y=z=s, and we are told that s=3

d^2=3s^2  and s=3

d^2=3(3^2)

d^2=3(9)

d^2=27

d=√27