A book has w chapters. The first chapter is w5 pages long the second chapter is y pages long and the third chapter is w pages long. How many more pages does the second chapter have than the third chapter?
Let's denote the number of pages in the second chapter by p2 and the number of pages in the third chapter by p3.

p2 = w5
p3 = w

The difference in pages between the second and third chapter is p2 - p3.

Plugging in for p2 and p3 we get that the difference is w5 - w.

Therefore, the answer (symbolic expression) is: 

w5 - w