Answer
$5x^{2n}+19x^ny^n-4y^{2n}$
Work Step by Step
I use the FOIL method:
$(5x^n-y^n)(x^n+4y^n)=5x^{2n}+20x^ny^n-x^ny^n-4y^{2n}=5x^{2n}+19x^ny^n-4y^{2n}$
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