As functions of a, c, px, py , and I, write your demand for good x (i.e., the quantity of x in your favorite bundle that you can afford), and your demand for good y (the quantity of y in your favorite bundle that you can afford). Note: neither of the demands, for x or for y, will actually have all five of the variables a, c, px, py , and I in them. In fact, some of those variables won't appear in either demand