Absolutely ! If you have two vectors with equal magnitudes and opposite directions, then one of them is the negative of the other. Their correct vector sum is zero, and that's exactly the magnitude of the resultant vector.
(Think of fifty football players pulling on each end of the rope in a tug-of-war. Their forces are equal in magnitude but opposite in sign, and the flag that hangs from the middle of the rope goes nowhere, because the resultant force on it is zero.)
This gross, messy explanation is completely applicable when you're totaling up the x-components or the y-components.
