a) Here
b) Angle AGD is congruent to Angle KGL as they are vertical angles. AD is parrelel to KL because they both are perpendicular to the same line. Because AD is parrelel to KL we know by Alternate Interior Angles that ADG is congruent to GLK. Since we know two angles are congruent, by AA similarity, the triangles are similar.
c) DG/GK = 2 because we already proved that AD/LK = 2. They are the sides of a similar triangles, so the sides are proportional.
Not sure how D, G, and K are collinear. CPhill help me!!! (I have to prove that the angles formed by DG and GK is 180 by I can't find a way!)