You have to find the previous moves to move 6... where a piece for white moves from b4 to a3 to give mate. You don't know in this case which piece it might be or whether it takes a piece on a3. There is only one possible sequence of previous moves. There will be no situations where a pawn could have moved 1 or 2 spaces or a piece moves from X to Z but could have gone via Y1 or Y2. The exact moves are forced in the exact sequence. I generally try to find the mating position first then work backwards. Be warned.. it is addictive.