The sum of 5x and 9x can be found by adding them together. (Hence sum.)
Since they have like terms, they can be added with simple addition (you just have to place the X where it belongs.)
As so:
5x + 9x = ?
5 + 9 = 14, but we have an X as a variable in front of both 5 and 9, so we must put the X in front of our total of 5 & 9.
5x + 9x = 14x
That is our answer!
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