How do I solve 12x^2-89x-56 factor by grouping

Use ac method

multiply 12 and - 56

-672

now find what 2 factors of - 672 add to - 89

answer is - 96 and 7

expand the middle term like that

12x²-96x+7x-56

group

(12x²-96x) + (7x-56)

factor

12x (x-8) + 7 (x-8)

undistribute the (x-8)

(x-8) (12x+7) is answer