Go through your list of legal moves and check each move made by Sylvia.
Removing the same amount from both sides is a legal move.
Flipping tiles from the subtraction region to the addition region and changing it from positive to negative is a legal move.
Removing zero pairs 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.