So the way I've been taught is to use an X diagram. Multiply 4 and -84, then write the product on the top of the X. Then take your middle term and write it on the bottom, so it would be -336 on top and 16 on the bottom. Now try to find what multiplies to -336 and adds up to 16. I found 28 and -12 work, so now write out 4n^2+28n-12n-84, then split them so you have basically two equations. 4n^2 +28n and then -12n-84. Now factor each one of these individually, so you have 4n(n+7), -12(n+7). So your final answer is (4n-12)(n+7)
