I need help with number 11 Beginning with the quadratic equation y=ax^2+bx+c, Fred calculated -b/2a and then plugged that number in for x to get a Y
What is one reason he might have done this

The distribution of a sample of the outside diameters of pvc pipes approximates a symmetrical, bell-shaped distribution. the arithmetic mean is 14.0 inches, and the standard deviation is 0.1 inches. about 68% of the outside diameters lie between what two amounts?