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:

$$ \color{blue}{ b = 4 } ~ \text{ and } ~ \color{red}{ c = -21 }$$

Now we must discover two numbers that sum up to $ \color{blue}{ 4 } $ and multiply to $ \color{red}{ -21 } $.

Step 2: Find out pairs of numbers with a product of $\color{red}{ c = -21 }$.

PRODUCT = -21
-1     21	1     -21
-3     7	3     -7

Step 3: Find out which pair sums up to $\color{blue}{ b = 4 }$

PRODUCT = -21 and SUM = 4
-1     21	1     -21
-3     7	3     -7

Step 4: Put -3 and 7 into placeholders to get factored form.

$$ \begin{aligned} x^{2}+4x-21 & = (x + \color{orangered}{\square} )(x + \color{orangered}{\square}) \\ x^{2}+4x-21 & = (x -3)(x + 7) \end{aligned} $$

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