<p>Coordinate S is (1.5, 4), therefore the center of the circle is (1.5, 0)</p>
<p>The distance between S, and the center of the circle is 4. (Distance formula).</p>
<p>Now the X coordinate is required from point R, which is all we can provide anyway. Since S, and the center is a radius and its length is equal to 4, therefore the distance between the center of the circle and R must also equal 4.</p>