Answer:
29.4 cm
Explanation:
The length of the space diagonal can be found to be the root of the squares of the three orthogonal edge lengths. For a cube, those edge lengths are all the same, so the diagonal length is ...
d = √(17^2 + 17^2 +17^2) = 17√3 ≈ 29.4 . . . . cm
_____
Consider a rectangular prism with edge lengths a, b, c. Then the face diagonal of the face perpendicular to edge "a" has length ...
(face diagonal)^2 = (b^2 +c^2)
and the space diagonal has length ...
(space diagonal)^2 = a^2 + (face diagonal)^2 = a^2 +b^2 +c^2
So, the length of the space diagonal is ...
space diagonal = √(a^2 +b^2 +c^2)
when the prism is a cube, these are all the same (a=b=c). This is the formula we used above.