Hello!
You can use the Pythagorean Theorem
[tex]a^{2} + b^{2} = c^{2} [/tex]
c is the diagonal length
a and b are the other side lengths
Put in the values you know
[tex]24^{2} + 45^{2} = c^{2} [/tex]
Square each of the numbers
[tex]576 + 2025 = c^{2} [/tex]
Add
[tex]2601 = c^{2} [/tex]
Take the square root of both sides
c = 51
The answer is 51
Hope this helps!