Nothing is said about the goalies. So, provided 2 of the 20 that we already have counted can also play as goalies, we simply have these 20 as our minimum size team.
b) Of the twelve who can play forward, 3 can play defence. So there are another 5 who play defence but not forward. That's 12+5=17 players so far.
Again, nothing has been said about the goalies. So provided two of these 17 can play as goalies, 17 is the minimum size of the team. (I therefore disagree with the answer of 19 that you have been given.)
c) The 2 goalies can play forward but not defence. So the 12 who can play forward are all we need to account for the forwards and the goalies. Provided, then, that 8 of the 10 forwards who can't play in goal can play in defence, 12 is all we need to meet the criteria. (Again, that differs from the answer in the back of the book.)