Aysah wants to vend tea at the Wednesday Wellness Fair. She buys a pitcher and sachet tea bags from a store for this purpose. While vending, the cost for each cup of will be the same. If Aysah sells 48 cups of tea, she will have a profit of $144. If she sells 20 cups of tea, she will have a profit of $60.

Determine the rate of change for a function that represents Aysah's profit, y, in dollars, from selling x cups of tea.