Question 315682
{{{5x+7x=12x}}}
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. 
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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.