Can someone please explain to me the steps slowly how to factorise cubics??

The question is Factorise (s^2 - 3s + 2)(s+4)

And the answer is (s-1)(s-2)(s-4)

I just don't get how they got from the first step to the last step.. Here's what I tried to do...

s^3 + 4s^2 -3s^2 -12s + 2s +8

s^3 + s^2 -10s + 8

s(s^2 + s - 10) + 8

And this is where i don't know what to do next ><

Can someone please help me????