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#### bkelly2442

##### New member

- Jun 28, 2016

- 7

I'm helping a friend's daughter factoring trinomials. I can get to the correct answer, but usually thru a system of trial and error and not a defined set of rules in every instance. Example

6x^2 + 39x - 21

3(2x^2 +13x-7)

Here's where I get stuck. I know that the factors of -7 are going to be 1, -7 or -1, 7. However, these don't add to 13. I suppose that has something to do with the factored out 3? Or the unkown? Also, when I do get the 2 sets of binomials, what is the rule to determine whether I use 1, -7 or -1, 7? What is the rule that determines if my sets are (2x-1)(x+7) or (x-1)(2x+7) or (2x+1)(x-7) or (x+1)(2x-7) or (2x-7)(x+1) and so on and so on and so on......