Answer:
The easiest way to calculate a diagnol is with the pythagorean theorem if A is the length of the hypotenuse, and B and C are the legs of a right triangle:
A = (B^2+C^2)^.5
If you are given certain angle(s) and and a side length, you can use trigonometry to find a diagnol.
Note: correct spelling is D I A G O N A L.