Try this puzzle

mate with rook and knight only

There is only 1 solution

that is a help puzzle..     fools mate is shortest for help puzzles

mate with rook and knight only 

and whats a help puzzle

Helpmate puzzles (or generalized to help[whatever] puzzles when the objective is not to mate) are puzzles where both sides cooperate to achieve the objective, which is what you're doing (White and Black cooperate to mate Black).

"Rook and knight" only is not clear, since after all Black's d- and f-pawns, queen, and dark bishop play in the role of blocking other flight squares. However, here's one that you will most likely accept as it uses the same mating pattern:

For White, it's this

a help puzzle is where 1 side plays wrong moves (such puzzles are just to check the shortest possible checkmate or something like that)

you got it

oh, Interesting