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 $\color{blue}{ a = 24 }$ by the constant term $\color{blue}{c = -15} $.
$$ a \cdot c = -360 $$Step 3: Find out two numbers that multiply to $ a \cdot c = -360 $ and add to $ b = 26 $.
Step 4: All pairs of numbers with a product of $ -360 $ are:
PRODUCT = -360 | |
-1 360 | 1 -360 |
-2 180 | 2 -180 |
-3 120 | 3 -120 |
-4 90 | 4 -90 |
-5 72 | 5 -72 |
-6 60 | 6 -60 |
-8 45 | 8 -45 |
-9 40 | 9 -40 |
-10 36 | 10 -36 |
-12 30 | 12 -30 |
-15 24 | 15 -24 |
-18 20 | 18 -20 |
Step 5: Find out which factor pair sums up to $\color{blue}{ b = 26 }$
PRODUCT = -360 and SUM = 26 | |
-1 360 | 1 -360 |
-2 180 | 2 -180 |
-3 120 | 3 -120 |
-4 90 | 4 -90 |
-5 72 | 5 -72 |
-6 60 | 6 -60 |
-8 45 | 8 -45 |
-9 40 | 9 -40 |
-10 36 | 10 -36 |
-12 30 | 12 -30 |
-15 24 | 15 -24 |
-18 20 | 18 -20 |
Step 6: Replace middle term $ 26 x $ with $ 36x-10x $:
$$ 24x^{2}+26x-15 = 24x^{2}+36x-10x-15 $$Step 7: Apply factoring by grouping. Factor $ 12x $ out of the first two terms and $ -5 $ out of the last two terms.
$$ 24x^{2}+36x-10x-15 = 12x\left(2x+3\right) -5\left(2x+3\right) = \left(12x-5\right) \left(2x+3\right) $$