Answer:
What you are looking for is the length of the 'solid diagonal'. First we need to find the length of the base diagonal. That can be done easily using Pythagorean theorem. 52+32−−−−−−√=6. Now this diagonal is a leg of the bigger right triangle whose hypotenuse is the required solid diagonal. You can see this from amd's picture. So apply Pythagorean theorem again. 62+(2.5)2−−−−−−−−−√=37.25−−−−√. You can work that out with a calculator.
In general for a cuboid with dimensions a, b and c(order doesn't matter), the solid diagonal is: a2+b2+c2−−−−−−−−−−√
Explanation: