Ans: 13 cm
In order to find the diagonal we need the length of the two other sides. One of those is twelve, the other is unknown; we’ll call it x.
Side X cuts the bottom the rectangular prism in half by the diagonal, this creates two triangles with sides 4, 3, and x.
X is the hypotenuse, meaning we can use Pythagorean theorem (a^2 +b^2 = c^2)
4^2 + 3^2 = x^2
25 = x^2
5=x
The height of the diagonal triangle is 12 and the base is 5.
We can use the Pythagorean theorem again
12^2 + 5^2 = c^2
144+25 = c^2
Square root of 169 = c
13 = c