Solve x^2=18 where x is a real number

Use the inverse of the function y=x^2-18x to find the unknown value [tex]y = \sqrt{bx + c \: + d} [/tex]

Simplify a7/a4 a) 1/a3 b)-a/a2 c)a4/a3 d)a3

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! )