This is just good 'ol Pythagorean Theorem.
a^2 + b^2 = c^2
The diagonal will be c because c is always across from the right angle of a right triangle.
3.6^2 + 2.4^2 = c^2
12.96 + 5.76 = c^2
18.72 = c^2
Now take the square root of 18.72 to find c. You get 4.32666...
