Idon't get it. do i start inside the parentheses?

How do i do this? i don’t understand

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

Cos^2x+cos^2(120°+x)+cos^2(120°-x)i need this asap. pls me