what is the Ceofficient of x^4 in the Expansion of (1+X)(1-x) and for what values of the integers n will the term involving x^4 be abscent ​

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! )