Lucy went to the grocery store. She bought w packs of cookie and x packs of cake. How many more packs of cookie did she buy over cake?
Let's denote the number of packs of cookies by c and the number of packs of cake by k.

c = w
k = x

The difference between the number of packs of cookies and the number of packs of cake is c - k.

Plugging in for c and k we get that the difference is w - x.

Therefore, the answer (symbolic expression) is: 

w - x