题目计算:(1)(m−2n)(m+2n)−m(m−2n)(1)\left(m-2n\right)\left(m+2n\right)-m\left(m-2n\right)(1)(m−2n)(m+2n)−m(m−2n)；(2)(y+1−3y+3)÷y2+8y+16y2+3y(2)(y+1-\frac{3}{y+3})÷\frac{y^{2}+8y+16}{y^{2}+3y}(2)(y+1−y+33)÷y2+3yy2+8y+16.知识点：解二元一次方程组——代入消元法章节：方程与不等式 / 二元一次方程组
