If one of the two factors of an expression which is the difference of two cubes is (x4 + x2y + y2), then the other factor would be -
x4 + x2y + y2 = (x2)2 + x2y + (y)2
This is in the form of a2+ ab + b2 where a = x2, b = y
The other factor is a – b as a3 – b3 = (a - b)( a2+ ab + b2)
Hence, other factor of x4 + x2y + y2 is x2 - y