They decided to hold the party in their backyard. If they have 11 sets of tables and each set has 13 chairs how many chairs do they have in the backyard?

There are two chairs in the front yard. 
We know this because the question says, "In the front yard, there are two chairs." 

We also know that there is a porch with four chairs. 
We know this because the question says, "On the porch, there are four chairs." 

Now, we just need to add two and four to get our answer. 

There are six chairs in the backyard.