For this question, sqrt(x^2-36)=8 have the form of square root which is not a good form to solve the question. So, first, we try to take square of both sides to eliminate the square root.
(sqrt(x^2-36))^2=(8)^2 x^2-36=64 x^2-100=0 (x+10)(x-10)=0 x=10 or x=-10