x^2 - 81 is a perfect square.
x^2 - 81 = 0
bring over -81 (but change the sign)
x^2 = 81
then take the square root of 81. when you do this, you will get one answer; however, you must take both the positive and negative value (because it is a perfect square)
sqrt 81 = 9
therefore x = 9, x = -9