As angles BAC and BDA are right angles angle BAD is congrunet to to angle ACD (amoung other reasons because triangles BAD and BAC are both right and have a common angle other than the right angle)
Hence triangles ABD and CAD are both right triangles and another pair of congruent andgles and so are similar.
Part (b) you do by forming ratios of corresponding sides of two of the similar triangles.
Part (c) yuo use the result of part (b) and Pythogoras' theorem to find AD, then the area is (1/2)AD.BD and BD is given.
RonL