Hi :) in the pic is a question that i need help with i just dont get it

How can you decide witch operations to use to slice a two step equation

Suppose you are an avid reader and are looking to save money on the cost of books instead of paying about $20 for each book, you decide to look into purchasing a kindle for $120 you can purchase e-books for about $5 each 1. find the slope-intercept equation for the cost of the hard copy of books using x to represent the number of books 2. find the slope-intercept equation for the cost of the kindle with cost of e-books using x to represent the number of books

Find the value of x. give reasons to justify your solutions! c ∈ ae