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 $\color{blue}{ a = 3 }$ by the constant term $\color{blue}{c = -24} $.
$$ a \cdot c = -72 $$Step 3: Find out two numbers that multiply to $ a \cdot c = -72 $ and add to $ b = 14 $.
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 $\color{blue}{ b = 14 }$
PRODUCT = -72 and SUM = 14 | |
-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 $ 14 x $ with $ 18x-4x $:
$$ 3x^{2}+14x-24 = 3x^{2}+18x-4x-24 $$Step 7: Apply factoring by grouping. Factor $ 3x $ out of the first two terms and $ -4 $ out of the last two terms.
$$ 3x^{2}+18x-4x-24 = 3x\left(x+6\right) -4\left(x+6\right) = \left(3x-4\right) \left(x+6\right) $$