The square of the diagonal (d1) of the end face is
.. (d1)^2 = (3 in)^2 +(5 in)^2 = (9 +25) in^2 = 34 in^2
The square of the longest diagonal (d2) will be given by
.. (d2)^2 = (3x in)^2 +(d1)^2
.. = (9x^2 +34) in^2
So, the longest diagonal is
.. d2 = √(9x^2 +34) in