draw a segment parallel to CD from point B to AC ... let the intersection point with AC be point E.
right triangle ABE ... vertical leg = a-b , horizontal leg = x , hypotenuse = a+b
now use Pythagoras to get the desired result.
... btw, next time start a new problem w/ a new thread.
(To Prove It) If you read my original message, sorry, didn't read properly, must be tired xD
cw86, the Pythagorean Theorem allows you to recover the length of one side of a right angled triangle provided the two other sides. Now, plugging in the values of "a" and "x" in the equation and solving for "b", you will have your answer
Does it make sense ? Or do you need further help ?