Or another way is to memorize that the center and bottom of an upward facing parabola is at location x=4 because it is one half and negative of the minus eight x. Then substitute 4 for x and determine how far below the x-axis the bottom of the parabola is. Hence y=16-32+5 or negative eleven. Then since the parabola is normal width of one (one x squared term), then take the square root of the eleven height and get the width in either direction on the x-axis. So the parabola crosses the x-axis at about 0.683 and 7.317. If the parabola's x squared term was two, then the parabola would probably be skinnier since y gets twice as big. So in that case, you'd probably take the eleven(distance bottom of parabola is from x-axis) in this example and divide it by two before square rooting it. I just made this all up, so enjoy it.