Binomials are expressions with only two terms being added.
2x^2 - 4x is an example of a binomial. (You can say that a negative 4x is being added to 2x2.)
First, factor out the GCF, 2x. You're left with 2x (x - 2). This is as far as this binomial can go. Any binomial in the form 1x +/- n cannot be factored further.