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 $\color{blue}{ a = 56 }$ by the constant term $\color{blue}{c = 56} $.
$$ a \cdot c = 3136 $$Step 3: Find out two numbers that multiply to $ a \cdot c = 3136 $ and add to $ b = -113 $.
Step 4: All pairs of numbers with a product of $ 3136 $ are:
PRODUCT = 3136 | |
1 3136 | -1 -3136 |
2 1568 | -2 -1568 |
4 784 | -4 -784 |
7 448 | -7 -448 |
8 392 | -8 -392 |
14 224 | -14 -224 |
16 196 | -16 -196 |
28 112 | -28 -112 |
32 98 | -32 -98 |
49 64 | -49 -64 |
56 56 | -56 -56 |
Step 5: Find out which factor pair sums up to $\color{blue}{ b = -113 }$
PRODUCT = 3136 and SUM = -113 | |
1 3136 | -1 -3136 |
2 1568 | -2 -1568 |
4 784 | -4 -784 |
7 448 | -7 -448 |
8 392 | -8 -392 |
14 224 | -14 -224 |
16 196 | -16 -196 |
28 112 | -28 -112 |
32 98 | -32 -98 |
49 64 | -49 -64 |
56 56 | -56 -56 |
Step 6: Replace middle term $ -113 x $ with $ -49x-64x $:
$$ 56x^{2}-113x+56 = 56x^{2}-49x-64x+56 $$Step 7: Apply factoring by grouping. Factor $ 7x $ out of the first two terms and $ -8 $ out of the last two terms.
$$ 56x^{2}-49x-64x+56 = 7x\left(8x-7\right) -8\left(8x-7\right) = \left(7x-8\right) \left(8x-7\right) $$