原式= -(m+3)(m²+2m-3)/2+m(m²+2m-6)/2 = -3(m²+2m-3)/2-m(m²+2m-3)/2+m(m²+2m-6)/2 = -3(m²+2m-3)/2-3m/2 = -3(m²+3m-3)/2
