Factor completely: 4x^3 - 36x

You would take out a 4x out of both terms because this is a common factor 
4x^3-36x
4x(x^2-9) 
and then you would notice that (x^2-9) is a difference of squares, so that factors out too 
4x(x-3)(x+3)
that is all you can factor! hope this helps

The answer would be quadratic trinomial