Please help me, i really don't understand this

For each proportion find the unknown number x 1/5/8=×/15

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

If f(x) = 6x – 4, what is f(x) when x = 8? a2 b16 c44 d52