Go through your list of legal moves and check each move made by Sylvia.
Flipping tiles from the subtraction region to the addition region and changing it from positive to negative is a legal move.
Removing a positive tile from the (-) region and a negative tile from the (+) region is not a legal move, because they do not make a zero pair.
Removing the same amount from both sides is a legal move.
Removing zero pairs is a legal move.