See it is very using Stewarts theorem
http://en.wikipedia.org/wiki/Stewart's_theorem
By stewarts theorem we get
b2a2 + c2a2 = a( AD2+ a2 . a2 )
Putting values and solving
73 = AD2 + a24
Adding 25 (i.e b2) to both sides we get
98= AD2 +b2 + a24
98 = a24 + a24
a2 = 196
a = 14 units
:)