删除数组中的元素而不对索引进行硬编码？在斯威夫特

This is my first post, and I am very happy to join in this community. I am learning Swift with Ray Wenderlich's video tutorial. The challenge I got for lesson 3 is remove a element in an array without hard-coding the index. I understand the correct answer that Ray provided, but just don't understand why my answer can not work. Please see following for Ray's answer as well as my answer. If anyone can explain it for me, that would be great!! thanks:]This is my first post, and I am very happy to j