Pawn endgames are the most concrete endgames: the result can usually be calculated. Once the pieces are traded, an outside passed pawn becomes very strong because it drags the enemy king away.
White's a-pawn is an outside passed pawn. The rook endgame is good but not a sure win. Find the move that goes into a winning pawn endgame.
Rule: before trading into a pawn endgame, calculate it to the end. If the calculation is right, the trade is the shortest way to win; if it is wrong, there is no way back.