1/(6x-4y)+(1/6x+4y)-3x/(9x^2-4y^2)
=[(6x+4y)+(6x-4y)]/[(6x+4y)(6x-4y)]-3x/(9x^2-4y^2)
=12x/(9x^2-4y^2)-3x/(9x^2-4y^2)
=9x/(9x^2-4y^2)