Try and solve this puzzle ( checkmate in 1, black moves )
Type the FEN here! 8/8/8/8/8/8/1r6/r3k2K w - - 0 1
The FEN you provided is flawed. The only mate in 1 in the position for black would be if black castled however, your FEN has black's king on the first rank...instead of the 8th rank.
The proper FEN is this:
K3k2r/6r1/8/8/8/8/8/8 b k - 0 1