If AC = 36 and BC = 18, find AD. Round to the nearest tenth, if necessary.

Using: a^2 + b^2 = c^2 (pythagoras theorem)

where 'a' and 'b' are the legs of a right triangle, c is the hypotenuse

compare the triangle I just drew to the triangle you have for your problem

you'll see that a = unknown b = 18 c = 36

a=18sqrt3