You have a hub (benchmark) elevation of 102.20; you read 5.12 to the hub (backsite); you have a required elevation in the ditch bottom of 93.50; you read an existing grade of 11.50; what is the difference between the existing and the required?

You have a hub (benchmark) elevation of 102.20; you read 5.12 to the hub (backsite); you have a required elevation in the ditch bottom of 93.50; you read an existing grade of 11.50; what is the difference between the existing and the required?