PLS HELP I PROMISE ILL GIVE U BRAINLEST

2x minus y equals 6, x plus y equals negative 3

Find the maximum value of p= 4x + 5y subject to the following constraints :

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! )