AC=2X BC= under the root number (under the root number 2)? -X？ = 40-X under the root sign?

CD=BD= 40-X under the root sign? Divide the whole by 2.

CD？ +AC=AD？

(40-X under the root sign? Divided by 2)? +(2X)？ =5?

You should be able to solve this problem by yourself.

Then bring it in and work out AB by Pythagorean theorem.

I gave you the steps, and you have to pass on other formulas.