Abigail has $30.00 to buy groceries. She needs to buy some apples, some bananas, and 3 pounds of potatoes. Fuji apples are $1.79 a pound, bananas are $0.36 each, and potatoes are $2.99 for a 3 lb. bag. If a represents the number of pounds of apples Abigail buys and b represents the number of bananas she buys, which expression represents the amount of change she receives after her purchases?