1/6x-4y"+"1/6x+4y"-"2y/4y^2-9x^2"
=(6x+4y)/(36x^2-16y^2)+(6x-4y)/(36x^2-16y^2)+8y/(36x^2-16y^2)
=(12x+8y)/(36x^2-16y^2)
=(3x+2y)/(9x^2-4y^2)
=1/(3x-2y)