已知(m+n)乘以x的m次方乘以y的m-2次方乘以(3x乘以y的平方加上5乘以x的平方乘以y等于21乘以x的m次方乘以y的n+1次方加上35乘以x的m+1次方乘以y的n次方等于多少?????????有追加分!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!