x^3+3xy+y^3
=（x^3+y^3）+3xy
=(x+y)(x^2-xy+y^2)+3xy
x^2+y^2=(x-y)^2+2xy
=9+18=27
所以(x+y)(x^2-xy+y^2)+3xy
=(-3)*(27-9)+3*9
=-54+27
=-27
你的题可能是打错了 应该是y的立方