Find the coordinates of the point where the point has zero gradient

Chosen according to the volume of oil it can haul, as shown in at a railway yard, locomotives are used to haul containers carrying the table. locomotive capacity a450 0 -750 cubic feet cg35 750-1,500 cubic feet br73 1,500-2,500 cubic feet yh61 2,500-3,500 cubic feet & the four cylindrical containers listed in the table need to be hauled. cylinder length (ft.) diameter (ft.) fill level a 40 , 12 half b 24 8 full c 16 16 full 6 12 full d match each container to the locomotive needed to haulit a450 cylinder a yh61 cylinder d