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 $\color{blue}{ a = 9 }$ by the constant term $\color{blue}{c = -8} $.
$$ a \cdot c = -72 $$Step 3: Find out two numbers that multiply to $ a \cdot c = -72 $ and add to $ b = 6 $.
Step 4: All pairs of numbers with a product of $ -72 $ are:
PRODUCT = -72 | |
-1 72 | 1 -72 |
-2 36 | 2 -36 |
-3 24 | 3 -24 |
-4 18 | 4 -18 |
-6 12 | 6 -12 |
-8 9 | 8 -9 |
Step 5: Find out which factor pair sums up to $\color{blue}{ b = 6 }$
PRODUCT = -72 and SUM = 6 | |
-1 72 | 1 -72 |
-2 36 | 2 -36 |
-3 24 | 3 -24 |
-4 18 | 4 -18 |
-6 12 | 6 -12 |
-8 9 | 8 -9 |
Step 6: Replace middle term $ 6 x $ with $ 12x-6x $:
$$ 9x^{2}+6x-8 = 9x^{2}+12x-6x-8 $$Step 7: Apply factoring by grouping. Factor $ 3x $ out of the first two terms and $ -2 $ out of the last two terms.
$$ 9x^{2}+12x-6x-8 = 3x\left(3x+4\right) -2\left(3x+4\right) = \left(3x-2\right) \left(3x+4\right) $$