The absolute value of an expression is the non-negative (that is either 0 or positive) value of the expression without the sign.
For this problem, the absolute value of the expression (-x) must equal x (since x appears on the right side of the equation. This means that x is greater than or equal to 0. Thus, the equality is valid for all values of x greater than or equal to 9.
However, for negative values of x, the expression (-x) is positive. In this case, (-x) does not equal (x) so the equality |-x|=x is not valid.