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