In right triangle $ABC$ ( $BC$ is hypotenuse),$D$ is a point on $BC$.Calculate $AB$ given that... 
In right triangle $ABC$ ( $BC$ is hypotenuse),$D$ is a point on $BC$.Calculate $AB$ given that: $AD$=5,$BD=3$,$CD=6$.


You also can use Stewart's Theorem:
$$\frac{AC^2}{CD\cdot BC}-\frac{AD^2}{CD\cdot BD}+\frac{AB^2}{BD\cdot BC}=1\\ \frac{81-AB^2}{6\cdot 9}-\frac{5^2}{6\cdot 3}+\frac{AB^2}{3\cdot 9}=1$$
