Factorise 4a-16a-8ab?

- 12 a - 8 a b

- 4 a [ 3 + 2 b ]

Answer: -4a(3+2b)

first subtract 4a-16a since they have common variables to get -12a-8ab
Then factor out the a's since the 12 and 8 both have it as a variable to get a(-12-8b)
Then factor out the (-) sign since both have it as well to get -a(12+8b)
Then you can factor out a 4 out of the 12 & 8 to get -4a(3+2b)

And why should I do that?