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 $\color{blue}{ a = 15 }$ by the constant term $\color{blue}{c = 4} $.
$$ a \cdot c = 60 $$Step 3: Find out two numbers that multiply to $ a \cdot c = 60 $ and add to $ b = -23 $.
Step 4: All pairs of numbers with a product of $ 60 $ are:
PRODUCT = 60 | |
1 60 | -1 -60 |
2 30 | -2 -30 |
3 20 | -3 -20 |
4 15 | -4 -15 |
5 12 | -5 -12 |
6 10 | -6 -10 |
Step 5: Find out which factor pair sums up to $\color{blue}{ b = -23 }$
PRODUCT = 60 and SUM = -23 | |
1 60 | -1 -60 |
2 30 | -2 -30 |
3 20 | -3 -20 |
4 15 | -4 -15 |
5 12 | -5 -12 |
6 10 | -6 -10 |
Step 6: Replace middle term $ -23 x $ with $ -3x-20x $:
$$ 15x^{2}-23x+4 = 15x^{2}-3x-20x+4 $$Step 7: Apply factoring by grouping. Factor $ 3x $ out of the first two terms and $ -4 $ out of the last two terms.
$$ 15x^{2}-3x-20x+4 = 3x\left(5x-1\right) -4\left(5x-1\right) = \left(3x-4\right) \left(5x-1\right) $$