Triangle ADC is also a right triangle, with D the right angle, and AC the hypotenuse.
The triangles are similar (right triangles which share an angle (A), so...
AD/AC = AC/AB
5/6 = 6/AB
AB = 6*6/5 = 36/5 = 7.2
The answer to the first question is "C"
CD can be found using the Pythagorean theorem...
AD^2 + CD^2 = AC^2
25 + CD^2 = 36
CD^2 = 11
CD = sqrt(11) = 3.3