Solve the following quadratic equation by the square root property of equality.

x2 - 81 = 0

To solve for the square root property of equality of the quadratic equation, both sides of the equation must be square root. To do that we transfer the other term to the other side making it, x2 = 81. Now we can take the square root of both sides, √x2 = √81, x = 9 or x = - 9.