Notice that the second and the third term differentiate to zero in this case. It should be clear why the third term differentiated to zero. It’s a constant and we know that constants always differentiate to zero. This is also the reason that the second term differentiated to zero. Remember that since we are differentiating with respect to x here we are going to treat all y’s as constants. That means that terms that only involve y’s will be treated as constants and hence will differentiate to zero.