Which theorem do I use?
Answer question 2.

this may not be at all, but i use the same website you're using for school, and if i'm stuck doing one of these i try to look for it on the section.

the given 6 = w/p is equivalent to 6p = w, where w is the total area of the walls and p is the area covered by a single can of paint.

knowing that 6 cans of paint will be needed, 6p = w becomes 6(6) = w = 36.   this tells us that the total area to be painted is 36 square units.