题目计算:(x+y−3)(x−y+3)+(y+3)2\left(x+y-3\right)\left(x-y+3\right)+\left(y+3\right)^{2}(x+y−3)(x−y+3)+(y+3)2.知识点：单项式乘多项式章节：数与式 / 整式
