You can put this solution on YOUR website! Realize that y is a variable, meaning that any number can take its place. A good way to check if an answer is correct is by plugging in a random number to see if you get the same result. In 4y-4y, if you plug in 1, you get:
It is the same with any number. 4y and 4y are the same number no matter what you substitute in for y. Any number minus itself is always zero. Therefore, 4y-4y has to be 0.
