Let's look at this. We can see that both sides are equal.
x + 9 = 9 + xis the same thing asx + 9 = x +9
No matter what you put x to, they will be equal to each other. This means that x can equal anything, unless it is somehow not a number. For example, it cannot be infinity or negative infinity because it is not a number. This means that x can equal any real number or imaginary number.
