We have to use the Pythagorean theorem in this case which is
[tex]{a}^{2} + {b}^{2} = {c}^{2} [/tex]
where c is the hypotenuse. If we plug in the sides
[tex] {c}^{2} = {3}^{2} + {4}^{2} \\ {c}^{2} = 9 + 16 \\ {c}^{2} = 25 \\ c = 5[/tex]
The length of the diagnal is 5. It's important to notice that this is a 3 4 5 triangle.
