Question

In the figure below, what is the length of AD?

Answer 1

The answer will be H

Answer 2

So when finding what the side lengths of a triangle you need to use the Pythagorean Theorem. 

a² + b² = c²

You have three triangles in the picture above. ABC, ABD, and DBC

If I were to solve this I would find the measurements of Triangle DBC and use that to help with the others and to find out the measurement of line AD. 

I hope that helps a bit :)