Well you can use the Pythagorean theorem!
If the base is 15 and the length is 20 then the hypotenuse of the triangle then the hypotenuse is the square root of the squares added.
Thankfully this is actually a 3 - 4 - 5 triangle.
15 + 20 = c^2
3 + 4 = 5
So the diagonal is 25.
