Question

Plug in method?

Answer 1

You don't start with plugging in numbers. This is just testing whether you can find the right equation to plug numbers into. 

the equation would be k(2b-y): he has 2b books, they sell for k each--> k*2b, but y books did not sell for k, so it's now k(2b-y)

the answer is A but I hope you understand the process now.

Answer 2

Well, the picture is cut off below choice-C, and the whole thing is
very confusing to try and read because of trying to do all of your
work in the holes and spaces around the choices. So I'm just
going to ignore all of that, take the question, and work with it.

-- price of one book = k
-- total income if 1 book is sold = 1 · k
-- total income if 2 books are sold = 2 · k
-- total income if 100 books are sold = 100 · k

-- number of books for sale = 2b
-- total income if they're all sold = 2b · k
-- actual number that were sold = (2b - y)
-- actual total income = (2b - y) · k