Need help finding the value of X in this problem

Solve this system of equations. 12x − 18y = 27 4x − 6y = 10

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! )

Why is it so easy to buy on impulse and overspend with a credit card? what could you do to counteract this tendency?