This is a rectangle, meaning that all angles are right angles and all sides opposite to one another have equal length.
Since the length of CD is 24, the length of AB must also be 24.
Angle B is a right angle because this is a rectangle.
See anything? I'll you a hint. We have a right triangle with only one missing value.
What do we use to solve for missing lengths in a right triangle? The Pythagorean theorem, of course!
The length of c (the hypotenuse) is 26.
The length of a (one leg) is 24.
The length of b is unknown and we want to find it.
Subtract 576 by both sides.
Take square root of both sides.
The length of the missing leg is 10, meaning that the length of BD is 10.
This is what you should be needing. Enjoy!