Answer: B) like terms
======================================
Reason:
The 4x and -7x are like terms because they involve the same variable x. This allows us to combine the like terms to end up with -3x
4x-7x = -3x
-------
Another example of like terms is 2x and 5x.
Adding them gets us 2x+5x = 7x
We can think of it like saying "2 boxes + 5 boxes = 7 boxes". Replace each word "boxes" with the single letter "x" and we arrive at 2x+5x = 7x.