Nothing really to see.
ASCII Depiction:
|NNNN|
|---------| ---- square
|NNNN|
Turned into:
|n
|--------n
|__________n ----- Triangle
- I moved the cube into place, deleted the top right x vertices, leaving only 2 faces.
- Then I copied the bottom and moved it over 2y.
- Then I had to fiddle around with the dissolve options until I combined the bottom faces into one face and 4 edges/vertices.
- After that it was a fill job.