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:
Step 2: Multiply the leading coefficient $\color{blue}{ a = 2 }$ by the constant term $\color{blue}{c = 18} $.
$$ a \cdot c = 36 $$Step 3: Find out two numbers that multiply to $ a \cdot c = 36 $ and add to $ b = -15 $.
Step 4: All pairs of numbers with a product of $ 36 $ are:
PRODUCT = 36 | |
1 36 | -1 -36 |
2 18 | -2 -18 |
3 12 | -3 -12 |
4 9 | -4 -9 |
6 6 | -6 -6 |
Step 5: Find out which factor pair sums up to $\color{blue}{ b = -15 }$
PRODUCT = 36 and SUM = -15 | |
1 36 | -1 -36 |
2 18 | -2 -18 |
3 12 | -3 -12 |
4 9 | -4 -9 |
6 6 | -6 -6 |
Step 6: Replace middle term $ -15 x $ with $ -3x-12x $:
$$ 2x^{2}-15x+18 = 2x^{2}-3x-12x+18 $$Step 7: Apply factoring by grouping. Factor $ x $ out of the first two terms and $ -6 $ out of the last two terms.
$$ 2x^{2}-3x-12x+18 = x\left(2x-3\right) -6\left(2x-3\right) = \left(x-6\right) \left(2x-3\right) $$