x^4-13x^2y^2+36y^4
=x^4-4x^2y^2-9x^2y^2+36y^4
=x^2(x^2-4y^2)-9y^2(x^2-4y^2)
=(x^2-9y^2)(x^2-4y^2)
=(x+3y)(x-3y)(x+2y)(x-2y)