Implement opposite() method to tell if there are two opposite numbers, (x,-x)
Here is what they intended you to do. You keep a counter of the number of opposite elements, which starts at zero. When inserting an element $x$, you insert both the true element $x$ and the dummy element $-x$; if $-x$ was already there, then you increase your counter of opposite elements. Similarly, when deleting $x$ you delete both $x$ and $-x$, and possibly decrease the counter.