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:
$$ \color{blue}{ b = -15 } ~ \text{ and } ~ \color{red}{ c = 54 }$$Now we must discover two numbers that sum up to $ \color{blue}{ -15 } $ and multiply to $ \color{red}{ 54 } $.
Step 2: Find out pairs of numbers with a product of $\color{red}{ c = 54 }$.
PRODUCT = 54 | |
1 54 | -1 -54 |
2 27 | -2 -27 |
3 18 | -3 -18 |
6 9 | -6 -9 |
Step 3: Find out which pair sums up to $\color{blue}{ b = -15 }$
PRODUCT = 54 and SUM = -15 | |
1 54 | -1 -54 |
2 27 | -2 -27 |
3 18 | -3 -18 |
6 9 | -6 -9 |
Step 4: Put -6 and -9 into placeholders to get factored form.
$$ \begin{aligned} x^{2}-15x+54 & = (x + \color{orangered}{\square} )(x + \color{orangered}{\square}) \\ x^{2}-15x+54 & = (x -6)(x -9) \end{aligned} $$