Mary is baking a cake. The recipe calls for w cups of sugar and x cups of flour. She already put in y cups of flour and z cups of sugar. How many more cups of sugar does she need to add?
The recipe calls for w cups of sugar and x cups of flour.

Mary has already put in y cups of flour and z cups of sugar.

Therefore, she needs to add x - y cups of flour and w - z cups of sugar.

Therefore, the answer (symbolic expression) is: 

w - z