The two branches that go from H and H' are Dental disease (D) and no Dental disease (D'). Pr(D | H) = 0.78 and so Pr(D' | H) = 0.22. Also Pr(D | H') = 0.3 and so Pr(D' | H') = 0.7.
For part A the tree diagram shuold make it clear how to calculate Pr(H | D).
For part B, you first need to edit the tree diagram because now Pr(H) = 0.4.