Please show me hoe to calculate this. thank you

(b) [12 marks] Suppose the market demand only arises every second period. In other periods, demand is equal to zero. Construct a subgame perfect equilibrium with trigger strategies in which firms collude on the industry-profit maximising prices and punish deviation by reverting forever to the static Nash equilibrium. Under which condition does this equilibrium hold? Consider a market for a homogenous product with four active companies. Firms have a constant marginal cost of production of $10. The market demand is given by 13(1)) = 90 11 Firms set prices in a repeated game with infinite horizon and a discount factor 6 6 [0,1]