6√3 is the length of the cube's diagonal.

_____

The diagonal of the cube runs through the center of the cube between opposite corners. It is the hypotenuse of a triangle formed by one edge and the diagonal of one face.

6

d = √(36*3) = 6√3

_____

The diagonal of the cube runs through the center of the cube between opposite corners. It is the hypotenuse of a triangle formed by one edge and the diagonal of one face.

6

^{2}+ (6√2)^{2}= d^{2}= 36 + 36*2 = 36*3d = √(36*3) = 6√3