#### Answer

10.8 cm

#### Work Step by Step

First, we must find the length of the diagonal along the bottom of the cube:
$ a^2 + b^2 = c^2 \\ c = \sqrt{a^2 + b^2} \\ c = \sqrt{6^2 + 8^2} =10$
We now find the length of the diagonal from the lower to the upper corner:
$ c = \sqrt{a^2 + b^2} \\ c = \sqrt{10^2 + 4^2} = 10.8$