Answer
20.8 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{12^2 + 12^2} =16.97$
We now find the length of the diagonal from the lower to the upper corner:
$ c = \sqrt{a^2 + b^2} \\ c = \sqrt{12^2 + 16.97^2} = 20.8$
