7х²-14х-56=7*(х+2)(....)

7х²-14х-56= 7*(х²- 2х- 8)

х²- 2х- 8=0

D=4+32=36

x=(2+6)/2=4

x=(2-6)/2=-2

х²- 2х- 8=(x-(-2))(x-4) = (x+2)(x-4)

7х²-14х-56=7*(х+2)(x - 4)