Mary is baking a cake. The recipe calls for w cups of flour and x cups of sugar. She already put in y cups of flour. How many more cups of sugar than cups of flour does she need to add now?
The recipe calls for w cups of flour and x cups of sugar. Mary has already put in y cups of flour, which means she needs to add (w-y) cups of flour. She also needs to add x cups of sugar. 

This means that she needs to add (x-(w-y)) cups of sugar. 

Therefore, the answer (symbolic expression) is: 

x-(w-y)