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:

$$ a = 9 , b = -73 ~ \text{ and } ~ c = 70 $$

Step 2: Multiply the leading coefficient $\color{blue}{ a = 9 }$ by the constant term $\color{blue}{c = 70} $.

$$ a \cdot c = 630 $$

Step 3: Find out two numbers that multiply to $ a \cdot c = 630 $ and add to $ b = -73 $.

Step 4: All pairs of numbers with a product of $ 630 $ are:

PRODUCT = 630
1     630	-1     -630
2     315	-2     -315
3     210	-3     -210
5     126	-5     -126
6     105	-6     -105
7     90	-7     -90
9     70	-9     -70
10     63	-10     -63
14     45	-14     -45
15     42	-15     -42
18     35	-18     -35
21     30	-21     -30

Step 5: Find out which factor pair sums up to $\color{blue}{ b = -73 }$

PRODUCT = 630 and SUM = -73
1     630	-1     -630
2     315	-2     -315
3     210	-3     -210
5     126	-5     -126
6     105	-6     -105
7     90	-7     -90
9     70	-9     -70
10     63	-10     -63
14     45	-14     -45
15     42	-15     -42
18     35	-18     -35
21     30	-21     -30

Step 6: Replace middle term $ -73 x $ with $ -10x-63x $:

$$ 9x^{2}-73x+70 = 9x^{2}-10x-63x+70 $$

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

$$ 9x^{2}-10x-63x+70 = x\left(9x-10\right) -7\left(9x-10\right) = \left(x-7\right) \left(9x-10\right) $$

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