Answer:
8.7
Explanation:
formula
d = sqrt(s^2 + s^2 + s^2)
d = sqrt(3*5^2)
d = sqrt(75)
d = 8.7
Where did I get the formula? If the cube sits on one of its faces then the diagonal of that face is sqrt(s^2 + s^2) = sqrt(2s^2)
Now to get the third s^2, you have to realize the the diagonal you calculated in the previous remark is one of the legs for the cube's corner to corner diagonal. The other leg is the height squared which is also s^2.