=a^4-a^2b^3-a^2b^3+b^6
=(a^4-a^2b^3)-(a^2b^3-b^6)
=a^2(a^2-b^3)-b^3(a^2-b^3)
=(a^2-b^3)(a^2-b^3)
=(a^2-b^3)^2