Set up a system of equations describing the following word problem. Be sure to specify what each variable represents.
Jemima wants to make chocolate-chip walnut brownies. Chocolate chips come in a 12 oz bag that costs $3. Walnuts come in a 4 oz bag that costs $2. If Jemima needs three pounds of chocolate chips and walnuts combined, and has $15 to spend, how many bags of each can she buy? BTW, what's with the shoestring budget, Jemima? If you're going to start this chocolate-chip walnut brownie venture of yours, build up some capital and really go for it. Even if you're only entertaining a few guests.