1) First, you have to add all the 3 pieces of wood
10+7.5+12= 29.5 ft.
Second, you have to add the pieces that he has used
5.2 +6.2= 11.4 ft
Third, you have subtract the used wood from the whole wood to get how many feet he didn't use
29.5 - 11.4=18.1 ft.
I hope it helped. But, sorry don't know how to do number 2.