Step 1 :
After factoring out $ -2 $ we have:
$$ -2x^{2}+14x+36 = -2 ( x^{2}-7x-18 ) $$Step 2 :
Step 2: 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:
$$ \color{blue}{ b = -7 } ~ \text{ and } ~ \color{red}{ c = -18 }$$Now we must discover two numbers that sum up to $ \color{blue}{ -7 } $ and multiply to $ \color{red}{ -18 } $.
Step 3: Find out pairs of numbers with a product of $\color{red}{ c = -18 }$.
PRODUCT = -18 | |
-1 18 | 1 -18 |
-2 9 | 2 -9 |
-3 6 | 3 -6 |
Step 4: Find out which pair sums up to $\color{blue}{ b = -7 }$
PRODUCT = -18 and SUM = -7 | |
-1 18 | 1 -18 |
-2 9 | 2 -9 |
-3 6 | 3 -6 |
Step 5: Put 2 and -9 into placeholders to get factored form.
$$ \begin{aligned} x^{2}-7x-18 & = (x + \color{orangered}{\square} )(x + \color{orangered}{\square}) \\ x^{2}-7x-18 & = (x + 2)(x -9) \end{aligned} $$