Question

factor completely:

4x^3 - 36x

Answer 1

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

Answer 2

the factor of this is 4x(x^2-9)
then x^2-9 can be factored using sum and difference.
4x(x+3)(x-3)