One Saturday, a butcher sells a total of b pounds of beef at $6.00 per pound. She also sells some pork for $7.00. On Sunday she sells b pounds of beef again, but at the sale price of $4.00 per pound. She also sells some pork for $9.00. Given that she made the same revenue of d dollars each day, which of the following systems of equations can be used to find out how many pounds of beef, b, she sold each day?