As I had written in the post on the current slides, I am responding to Osezua Ibhadode’s questions.
Of course the solution here is direct and laborious. It is after you have gone through such labor that the patterns emerge: The fact that the Kronecker Delta is a “substitution operator” becomes clear. The final answer will be immediately obvious to you after you have fully understood that. That understanding will constantly elude any student who will not do this labor in the first instance.