Step 1: determine which information is relevant for the subquestion (and which information is not). Houses 13 through 18 are not directly south of any other house, so you can ignore these houses for the first subqestion, we're only interested here in houses 1 through 12.
Step 2: identify the variables. For the first subquestion there are two variables: the number of a northern house and the number of its opposing southern house. Let's call the number of a northern house , and the number of its opposing southern house
Step 3: make a formula that links the the two house numbers. In this case a simple linear formula will do:
I'll leave it up to you to figure out the values for constants and
Then repeat the same steps for the second subquestion.