As i said in another thread a good book with a lot of examples in formalizing every day sentences is: Logic by schaum's outline series

Problem (c) in your question is a classic formalization of the additive inverse in real Nos

Here formalization can be done in two ways:

1) If you mention the set in which you state your axioms and definitions,which in this case in concern is the set of real Nos,then we have:

2) if you do not define the set then you can include it in your formalization in the following way:

YOU can do the rest of the problems