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:

$$ a = 3 , b = -2 ~ \text{ and } ~ c = -8 $$

Step 2: Multiply the leading coefficient $\color{blue}{ a = 3 }$ by the constant term $\color{blue}{c = -8} $.

$$ a \cdot c = -24 $$

Step 3: Find out two numbers that multiply to $ a \cdot c = -24 $ and add to $ b = -2 $.

Step 4: All pairs of numbers with a product of $ -24 $ are:

PRODUCT = -24
-1     24	1     -24
-2     12	2     -12
-3     8	3     -8
-4     6	4     -6

Step 5: Find out which factor pair sums up to $\color{blue}{ b = -2 }$

PRODUCT = -24 and SUM = -2
-1     24	1     -24
-2     12	2     -12
-3     8	3     -8
-4     6	4     -6

Step 6: Replace middle term $ -2 x $ with $ 4x-6x $:

$$ 3x^{2}-2x-8 = 3x^{2}+4x-6x-8 $$

Step 7: Apply factoring by grouping. Factor $ x $ out of the first two terms and $ -2 $ out of the last two terms.

$$ 3x^{2}+4x-6x-8 = x\left(3x+4\right) -2\left(3x+4\right) = \left(x-2\right) \left(3x+4\right) $$

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