# How do you factor 6ab − 8a + 21b − 28?

Jul 18, 2016

$6 a b - 8 a + 21 b - 28 = \left(2 a + 7\right) \left(3 b - 4\right)$

#### Explanation:

Notice that the ratio between the first and second terms is the same as that between the third and fourth terms.

So this will factor by grouping:

$6 a b - 8 a + 21 b - 28$

$= \left(6 a b - 8 a\right) + \left(21 b - 28\right)$

$= 2 a \left(3 b - 4\right) + 7 \left(3 b - 4\right)$

$= \left(2 a + 7\right) \left(3 b - 4\right)$