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:

$$ a = 28 , b = -33 ~ \text{ and } ~ c = -28 $$

Step 2: Multiply the leading coefficient $\color{blue}{ a = 28 }$ by the constant term $\color{blue}{c = -28} $.

$$ a \cdot c = -784 $$

Step 3: Find out two numbers that multiply to $ a \cdot c = -784 $ and add to $ b = -33 $.

Step 4: All pairs of numbers with a product of $ -784 $ are:

PRODUCT = -784
-1     784	1     -784
-2     392	2     -392
-4     196	4     -196
-7     112	7     -112
-8     98	8     -98
-14     56	14     -56
-16     49	16     -49
-28     28	28     -28

Step 5: Find out which factor pair sums up to $\color{blue}{ b = -33 }$

PRODUCT = -784 and SUM = -33
-1     784	1     -784
-2     392	2     -392
-4     196	4     -196
-7     112	7     -112
-8     98	8     -98
-14     56	14     -56
-16     49	16     -49
-28     28	28     -28

Step 6: Replace middle term $ -33 x $ with $ 16x-49x $:

$$ 28x^{2}-33x-28 = 28x^{2}+16x-49x-28 $$

Step 7: Apply factoring by grouping. Factor $ 4x $ out of the first two terms and $ -7 $ out of the last two terms.

$$ 28x^{2}+16x-49x-28 = 4x\left(7x+4\right) -7\left(7x+4\right) = \left(4x-7\right) \left(7x+4\right) $$

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