Please show how to calculate. Thank you

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 DU?) = 90 :0- Firms set prices in a repeated game with infinite horizon and a discount factor (5 E [0,1]. (a) [12 marks] 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