If you start from (8, 1) and "trade 4 units of x for 1 unit of y", you get (8- 4, 1+1)= (4, 2) Saying that she is indifferent between those means that . If you start from (4, 4) and "trade in 1 unit of x for 2 units of y", you get (4- 1, 4+ 2)= (3, 6). Saying she is indifferent between those means . That gives you two equations to solve for the two unknown values and .