This is a false statement.
When you are solving using square roots, you need to be aware that answers can be both positive and negative. When we solve this, you see there are two possible answers.
x^2 - 9 = 0
x^2 = 9
x = +/- 3
While 3 is an answer, so is -3. If we square either of those numbers, we get 9, which will satisfy the equation.