Which is the length of the arc MPN expressed in terms of pie?

Arandom high school question but a hard one

The price of a particular stock is represented by the linear equation , where x represents the number of weeks the stock has been owned and y represents the price of the stock, in dollars. if this relationship continues, what is the price of the stock after it has been owned for 12 weeks? $92.55 $94.37 $100.52 $114.39

Find the value of the cosine for angle a