They decided to hold the party in their backyard. They have w sets of tables and each set has x chairs. There are also y chairs extra. How many chairs do they have for the guests?
First, we need to find out how many chairs are in each set. We know that each set has x chairs, so the total number of chairs must be wx. 

Now, we need to find out how many extra chairs there are. We know that there are y extra chairs, so the total number of chairs must be wx + y. 

This means that the total number of chairs for the guests is (wx + y).

Therefore, the answer (symbolic expression) is: 

wx + y