Please explain how to solve question number 5

Consider the following static Prisoner's Dilemma game. 5. Let both players adapt the following strategy: start with cooperation; cooperate if both players cooperated or both players defected in the previous period; defect if one player defected and another player cooperated in the premioas period. In other words, the punishment for deviating from cooperation only lasts for one period, and both players revert to cooperation immediately after. Is it a SPNE if 6 = 3/5? What if 6 = 1/3