The average cost per hour in dollars of producing x riding lawn mowers is given by the following.
Upper C overbar left parenthesis x right parenthesis equals 0.5 x squared plus 23 x minus 216 plus StartFraction 2600 Over x EndFractionC(x)=0.5x2+23x−216+
2600
x
​(a) Use a graphing utility to determine the number of riding lawn mowers to produce in
      order to minimize average cost.
​(b) What is the minimum average​ cost?