Step 1: Identify constants $ \color{blue}{ b }$ and $\color{red}{ c }$. ( $ \color{blue}{ b }$ is a number in front of the $ x $ term and $ \color{red}{ c } $ is a constant). In our case:

$$ \color{blue}{ b = 10 } ~ \text{ and } ~ \color{red}{ c = 9 }$$

Now we must discover two numbers that sum up to $ \color{blue}{ 10 } $ and multiply to $ \color{red}{ 9 } $.

Step 2: Find out pairs of numbers with a product of $\color{red}{ c = 9 }$.

PRODUCT = 9
1     9	-1     -9
3     3	-3     -3

Step 3: Find out which pair sums up to $\color{blue}{ b = 10 }$

PRODUCT = 9 and SUM = 10
1     9	-1     -9
3     3	-3     -3

Step 4: Put 1 and 9 into placeholders to get factored form.

$$ \begin{aligned} x^{2}+10x+9 & = (x + \color{orangered}{\square} )(x + \color{orangered}{\square}) \\ x^{2}+10x+9 & = (x + 1)(x + 9) \end{aligned} $$

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