Danny collects bottle caps and wrappers. He found w bottle caps and x wrappers at the park. Now he has y wrappers and z bottle caps in his collection. How many more wrappers than bottle caps does danny have now?
Danny has w + x wrappers and w + z bottle caps in his collection.

Now he has y wrappers and z bottle caps in his collection.

Therefore, he has y - (w + z) wrappers and w + z bottle caps in his collection.

Therefore, the answer (symbolic expression) is: 

y - (w + z) - (w + z) = y - 2(w + z)