I do believe that's x MINUS y.
I'll try explaining it another way.
x = y is what the equation started out as.
x - y = 0 since x is always equal to y, subtracting the y is essentially the same as subtracting a number from itself which always gives you 0.
I can replace x - y with 0 because they equal each other.
So by dividing out the x - y, he was really dividing by 0 which broke the problem.