Subtract the Matrices A-B

In what form is the following linear equation written y=9x+2

Which ordered pair (c, d) is a solution to the given system of linear equations- c+2d=13 -9c-4d=-15

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! )