Can you kindly help me out please?!

If i have 20$ and i gave 5$ away how much i got

If cos(2x) = tan^2(y), show that cos(2y) = tan^2(x).

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