Answer
$$x = 6\tan \theta $$
Work Step by Step
$$\eqalign{
& {\text{The integral }}\sqrt {{x^2} + 36} {\text{ contains }}{x^2} + {a^2},{\text{ then the change of }} \cr
& {\text{variables suggested is}}:\,\,x = a\tan \theta \cr
& {\text{Let }}a = 6,{\text{ then}} \cr
& x = 6\tan \theta \cr} $$
