What is the area of the triangle below?

I'm confused on this could anyone me and show me what i need to do to solve it.

I'm at a bookstore with $\$35$ in my pocket and a $20\%$-off coupon. what is the cost of the most expensive book i can buy with my money and the coupon? (assume that there are books of every possible price in the bookstore. it's a large bookstore! )

Asequence is a function whose is the set of natural numbers