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#