Respuesta :

You have to square the binomial.
(x^3 + 4y)  (x^3 + 4y)  
Using FOIL:
x^9 + 4(x^3)y + 4(x^3)y + 16y^2
x^9 + 8(x^3)y + 16y^2

You could also use the formula: (a+b)^2 = (a^2 + 2ab + b^2).