(7x-2y)(3x-2y-1)-(4x+3y)(9x+6y) 求看的懂的 平方符号就用“平方”好了

(7x-2y)(3x-2y-1)-(4x+3y)(9x+6y)
=21x²-14xy-7x-6xy+4y²+2y-36x²-24xy-27xy-18y²
=-15x²-70xy-7x-14y²+2y怎么是70xy应该是71xy的嘛？不好意思，加错了，应该是：-15x²-71xy-7x-14y²+2yX=2,y=-1 麻烦再解一哈 然后我就给采纳和打赏-15x²-71xy-7x-14y²+2y=（-15）×4+71×2-14-14-2=52