Just like 5 oranges and 7 oranges give you 12 oranges or
5 cupcakes and 7 cupcakes gives you 12 cupcakes. It just happens that they're x's and not oranges or cupcakes. You treat them the same though.
Checking your answer is a good habit to get into.
It can save you from an embarassing error or verify that you didn't do something wrong in a complicated calculation.