13(x - 3)=39 what is answer

Which of the following is true for the relation f(x)=2x^2+1

Barney & noblet customers can choose to purchase a membership for $25 per year members receive 10% off all store purchases 1. how much would a member pay per year if he bought $50 worth of items each year? 2. write a slope-intercept equation that expresses how much a member pays per year using x to represent his purchases per year 3. how much would a member have to spend per year to pay off his membership fee?

Can someone me with #s 8, 9, and 11. with just one of the three also works. prove using only trig identities.