先用十字相乘法,然后再用提公因式法即可.
经观察,-18=3×(-6),所以可得：
x²-3xy-18y²-3x-9y
=(x²-3xy-18y²)-(3x+9y)
=(x+3y)(x-6y)-3(x+3y)
=(x+3y)(x-6y-3)