Need a little more explanation for calculating slack.

Problem 15-7 (Algo) Consider the following table of precedence relationships for a portion of a house building project: mon 00 No days Task Prepare Site B Install Rough Plumbing Pour Concrete Foundation D Concrete Curing Time E Preassemble Wall Frames F Erect Wall Frames G Install Roof H Install Wiring I Install Exterior Siding Install Insulation K Hang Drywall Windows M Paint Interior N Paint Exterior 0 Level Yard P Landscape Yard Duration Estimates 6 days 4 days 2 days 4 9 days 5 days 2 days 3 days 3 days 2 days 4 days 3 days 4 days 7 days 4 days 3 days NIN Immediate Predecessor(s) None Prepare Site Install Rough Plumbing Pour Concrete Foundation None Preassemble WF, Cure Concrete Erect Wall Frames Install Roof Install Roof Install Exterior Siding Install Insulation, Install Wiring Hang Dry Wall Install Windows Install Exterior Siding Cure Concrete Level Yard Task Earliest Start Immediate Predecessors None Earliest Completion 6 A 0 B A 6 10 C B 10 12 D C 12 16 E None 0 9 F DE 16 2 21 G F 21 23 H G 2 23 26 1 G 23 26 J 1 26 28 WIN K HJ 2 28 32 L 32 35 M L 35 39 N | | 26 33 O O D 16 20 O 2 20 23 Task Latest Start Slack Immediate Successors None Latest Completion 36 P 39 O P N None M None L M L J K 1 J.N H G HI G F YI OL G E F D F. O C D B A B