Answers

2014-09-18T19:38:47+08:00
-12xy+ (-15xy) = -27xy
-24xy+16xy =    -8xy
thus  -8xy - (-27xy)
  =  -8xy+27xy
  =  19xy
0
2014-09-19T13:02:16+08:00
I think the answer is -19xy. I'm not sure :)
0