Ok so this all about subtracting fractions this is what you want to do:
you can change 8 3/4 and 4 2/3 to have the same value and deominator 8 3/4 = 8 9/12(multiply both sides by 3) 4 2/3 = 4 8/12(multiply both sides by 4) you do this because 12 is divisible by 3 and 4
then, subtract 4 8/12 from 8 9/12 because John is using some of the wood his dad gave him. then subtract once again, taking 11/2 away from your difference. again match the deominator of 11/2 to the deominator of your difference which should be 12. another divisible of 12 is 2 and 6. 11/2 = 66/12 = 5 6/12(multiply both the top and bottom by 6) and start subtracting.