The way to tackle this question is to use the equation of a circle. The equation of a circle is x^2 + y^2 = r^2. Since you are given r and x, to find y you can simply rearrange this equation. So y = + or - sqrt(x^2+r^2). Now to find whether to use the positive or negative y, you can simply look at the picture. Since y is above the x-axis, it must be positive or y = sqrt(x^2+r^2). Let me know if you have any other questions!