This is how I do this exercise. No mouse required:

1. `cmd - shift - /` to jump to Help (works in any app)

2. type `tab` ["Hmm, I don't know exact shortcut to <undo close tab>, so let me start with something generic..."]

3. search through results with ctrl-n or ctrl-p.

4. As you search, different menu items show you the shortcut

And voila, your `cmd - shift - t` is the second result under "reopen closed tab"

EDIT: formatting bullets