Answer:
9x^8 + 6x^4y^2 + y^4
Step-by-step explanation:
(3x^4 + y^2)^2
= 3x^4(3x^4 + y^2) + y^2(3x^4 + y^2)
= 9x^8 + 3x^4y^2 + 3x^4y^2 + y^4
= 9x^8 + 6x^4y^2 + y^4
