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. In other words, there are w*x chairs in total for the sets of tables. 

There are also y chairs extra. 

Therefore, the total number of chairs they have for the guests is w*x + y. 

In this case, w*x + y = 12*14 + 6 = 168 + 6 = 174.