MISSED CHECK MATES, CAN YOU GET IT?

This game, Petroff defence, which i made a mistake very early in the game by move 6 where i moved my knight to Nc5. I had a cramped position. Another mistake at move 22, when i moved my pawn f7-f5 cost my knight and was in serious trouble. My opponent also made a mistake and allowed me to come back into the game. At move 41, i had the opportunity to exploit my opponents mistake and check mate him in few moves. But it was overlooked by me, and game ended in a draw. What is the best move for black after white moves his light squared bishop to a4 at move  41.