x4-2y4-3xy2+x2y+2xy2+3y4
=x4 -2y4+3y4 -3xy2+2xy2 +x2y
=x4+y4-xy2+x2y (合并同类项)
=x4+y4+x2y-xy2 (后两项换一下位置就行了)
=15-3
=12