 The Nash equilibrium (UA, X) is subgame perfect because it incorporates the subgame Nash equilibrium (A, X) as part of its strategy.To solve this game, first find the Nash Equilibria by mutual best response of Subgame 1.Then use backwards induction and plug in (A,X) → (3,4) so that (3,4) become the payoffs for Subgame 2.