we need to use pythagoras's theorem (reversed) in this situation. We know the hypotenuse and the lower line, so we can find the upright one. That will be the value of AD.
so we use 81 (9 squared) and 144 (12 squared)
let x be the upright line's length
144-81=x squared
63=x squared
find the square root of 63 and you can have your answer
Im not gonna do that for you because that's pretty easy
Use the straightedge to draw GK−→−. Place the point of the compass on point H and draw an arc in the interior of the angle. Label the intersection of the arcs in the interior of the angle as point K. Place the point of the compass on point G and draw an arc that intersects the sides of ∠G.