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:

$$ a = 3 , b = -20 ~ \text{ and } ~ c = -32 $$

Step 2: Multiply the leading coefficient $\color{blue}{ a = 3 }$ by the constant term $\color{blue}{c = -32} $.

$$ a \cdot c = -96 $$

Step 3: Find out two numbers that multiply to $ a \cdot c = -96 $ and add to $ b = -20 $.

Step 4: All pairs of numbers with a product of $ -96 $ are:

PRODUCT = -96
-1     96	1     -96
-2     48	2     -48
-3     32	3     -32
-4     24	4     -24
-6     16	6     -16
-8     12	8     -12

Step 5: Find out which factor pair sums up to $\color{blue}{ b = -20 }$

PRODUCT = -96 and SUM = -20
-1     96	1     -96
-2     48	2     -48
-3     32	3     -32
-4     24	4     -24
-6     16	6     -16
-8     12	8     -12

Step 6: Replace middle term $ -20 x $ with $ 4x-24x $:

$$ 3x^{2}-20x-32 = 3x^{2}+4x-24x-32 $$

Step 7: Apply factoring by grouping. Factor $ x $ out of the first two terms and $ -8 $ out of the last two terms.

$$ 3x^{2}+4x-24x-32 = x\left(3x+4\right) -8\left(3x+4\right) = \left(x-8\right) \left(3x+4\right) $$

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