原式=(a-1)(a-6)(a-3)(a+2)+56
=[(a-1)(a-3)][(a-6)(a+2)]+56
=[(a²-4a)+3][(a²-4a)-12]+56
令x=a²-4a
原式=(x+3)(x-12)+56
=x²-9x-36+56
=x²-9x+20
=(x-4)(x-5)
=(a²-4a-4)(a²-4a-5)
=(a²-4a-4)(a-5)(a+1)