I haven't worked with these in a very, very long time. Like maybe 7 years ago. I have no idea about the actual method anymore, but I remember enough that I see your problem with this one question.

As far as I can tell, you got the most important parts right, there's just one small detail involving the positive/negative sign. I'm pretty sure it should be (x+4).

As you have it right now, the negative signs would cancel each other out when multiplying the 3 and 4 at the end, and the 4x^2 would become -4x^2 instead. This could probably be explained better, but I hope it makes sense/helps as it is.