Factor polynomial $$ 3x^{2}-38x-13 $$

$$ 3x^{2}-38x-13 = ( x-13 )( 3x+1 ) $$

Step 1: Identify constants $ a $ , $ b $ and $ c $.
$ a $ is a number in front of the $ x^2 $ term $ b $ is a number in front of the $ x $ term and $ c $ is a constant. In this case:

$$ a = 3 , b = -38 ~ \text{ and } ~ c = -13 $$

Step 2: Multiply the leading coefficient $\color{blue}{ a = 3 }$ by the constant term $\color{blue}{c = -13} $.

$$ a \cdot c = -39 $$

Step 3: Find out two numbers that multiply to $ a \cdot c = -39 $ and add to $ b = -38 $.

Step 4: All pairs of numbers with a product of $ -39 $ are:

PRODUCT = -39
-1     39	1     -39
-3     13	3     -13

Step 5: Find out which factor pair sums up to $\color{blue}{ b = -38 }$

PRODUCT = -39 and SUM = -38
-1     39	1     -39
-3     13	3     -13

Step 6: Replace middle term $ -38 x $ with $ x-39x $:

$$ 3x^{2}-38x-13 = 3x^{2}+x-39x-13 $$

Step 7: Apply factoring by grouping. Factor $ x $ out of the first two terms and $ -13 $ out of the last two terms.

$$ 3x^{2}+x-39x-13 = x\left(3x+1\right) -13\left(3x+1\right) = \left(x-13\right) \left(3x+1\right) $$

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