Which of the following expressions is equal to sin(arccos(x/3))?

The answer is sqrt(9-x^2)/3. I am wondering why it is not "+ or -" sqrt(9-x^2)/3?

My rationale is that cos is positive in both the 1st and 4th quadrant so when I take sin of the angle represented by arccos(x/3) I should have to take the sin from both those quadrants.