Is it true 4 and -4 have a sum of 0

Yes, 4 + -4 = 0.

Think of it like a number line: The number four is 4 units to the right of the center which is 0, and the number negative four is 4 units to the left of the center. When you add them together, those distances cancel out, and you just get 0.

step by step explanation: when u removing the parentheses and combined them that would be : x+3y-x-y=2y