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?
First, we need to find out how many cups of sugar the recipe calls for. We know that the recipe calls for w cups of sugar and x cups of flour. This means that the recipe calls for w + x cups of sugar. 

Now, we need to find out how many cups of sugar Mary has already put in. We know that she has already put in y cups of flour and z cups of sugar. This means that she has already put in y + z cups of sugar. 

Now, we need to find out how many more cups of sugar Mary needs to add. To do this, we need to subtract the amount of sugar she has already put in from the amount of sugar the recipe calls for. We know that the recipe calls for w + x cups of sugar and Mary has already put in y + z cups of sugar. This means that Mary needs to add (w + x) - (y + z) cups of sugar. 

Therefore, the answer (symbolic expression) is: 

(w + x) - (y + z)