Question 192341
Opposites  are  just  as  you  stated.  Just  opposite  sign.  The  two  add  to  zero.

Normally  with  polynomials, we  talk  about  inverses.  This  is  the  reflextion  over  y=x.  We  usually  find  these  by  setting  x=y,  and  y=x.

For  example  if  y=2x,   inverse  is  x=2y  or  y=x/2.