Question 807145
{{{x^2+y^2=-2xy}}}
means


{{{(x+y)^2=0}}}

means 

{{{x+y=0}}}.

Now 

{{{x^3 + 2y^3=

(x^3 + y^3 )+y^3=

(x+y)(x^2-xy+y^2)+y^3=

(0)(x^2-xy+y^2) +y^3=

y^3}}}