I am not really sure about this type of problem, but a diameter has to cross the midpoint of the circle the dark dot and hit two point on both side of the circle. Yet the line 9.6 is not a diameter since it didn't go through the center, so the diameter has to be longer than 9.6. But x is looking for radius. so divide 9.6 by 2 u get 4.8 but that is not a diameter and the real diameter is longer that mean the radius is longer so the only answer is 5.4
So If you graph the function, you'll see that it never actually crosses the y axis into the third or fourth quadrant, the quadrants where all y values become negative.