Pls help i need this bad

How do you work out 21.3-8= show your working. plz

Adult wasps sting caterpillars and inject their eggs into the caterpillars. the larvae feed on the caterpillars, leading to the death of the caterpillars. what kind of a relationship exists between the wasps and the caterpillars? a. the adult wasps are parasites to the caterpillars. b. the caterpillars are parasites to the wasps. c. the adult wasps are commensals, and the caterpillars are hosts. d. the caterpillars are commensals, and the adult wasps are hosts.

Case # 51173878 weighs 814 pounds; case # 51173981 weighs 803 pounds; case # 51174058 weighs 817 pounds; case 51174178 weighs 815 pounds; case # 51174248 weighs 817 pounds; case # 51174495 weighs 781 pounds; case # 51174577 weighs 828 pounds. what are the case #'s of the two cases that weigh the same amount?