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 $\color{blue}{ a = 14 }$ by the constant term $\color{blue}{c = -18} $.
$$ a \cdot c = -252 $$Step 3: Find out two numbers that multiply to $ a \cdot c = -252 $ and add to $ b = -9 $.
Step 4: All pairs of numbers with a product of $ -252 $ are:
PRODUCT = -252 | |
-1 252 | 1 -252 |
-2 126 | 2 -126 |
-3 84 | 3 -84 |
-4 63 | 4 -63 |
-6 42 | 6 -42 |
-7 36 | 7 -36 |
-9 28 | 9 -28 |
-12 21 | 12 -21 |
-14 18 | 14 -18 |
Step 5: Find out which factor pair sums up to $\color{blue}{ b = -9 }$
PRODUCT = -252 and SUM = -9 | |
-1 252 | 1 -252 |
-2 126 | 2 -126 |
-3 84 | 3 -84 |
-4 63 | 4 -63 |
-6 42 | 6 -42 |
-7 36 | 7 -36 |
-9 28 | 9 -28 |
-12 21 | 12 -21 |
-14 18 | 14 -18 |
Step 6: Replace middle term $ -9 x $ with $ 12x-21x $:
$$ 14x^{2}-9x-18 = 14x^{2}+12x-21x-18 $$Step 7: Apply factoring by grouping. Factor $ 2x $ out of the first two terms and $ -3 $ out of the last two terms.
$$ 14x^{2}+12x-21x-18 = 2x\left(7x+6\right) -3\left(7x+6\right) = \left(2x-3\right) \left(7x+6\right) $$