Answer
$(x+y)(x^2-xy+y^2+1)$
Work Step by Step
The formula for factoring the sum of two cubes is: $A^3+B^3=(A+B)(A^2-AB+B^2)$.
Thus: $y^3+x+x^3+y=\\=y^3+x^3+x+y\\=(x+y)(x^2-xy+y^2)+(x+y)(1)\\=(x+y)(x^2-xy+y^2+1)$
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