<p>Its really a simple problem to visualize. It is a parabola. The vertex is at x = 20.</p>
<p>You want to find thex value of the other point that is equal to 10. So that means the y has to be the same. You are looking for the x value that has the same y value. Since a parabola has an axis of symmetry, you can just reflect it accross the axis to find the other point with the same y value.</p>