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?
There are w sets of tables and each set has x chairs. There are also y chairs extra. 

This can be rewritten as: 

There are w*x chairs in total for the sets of tables. There are also y chairs extra. 

Therefore, the total number of chairs for the guests is: 

w*x + y = 12*14 + 6 = 168 + 6 = 174 chairs.