（(x-2)(x-3)＋2(x+6)(x-5)＋3(x^2＋3x＋18)）/(－3x)
=[x^2-5x+6+2x^2+2x-60+3x^2+9x+54]/(-3x)
=(6x^2+6x)/(-3x)
=-2x-2