DSE 2017 Question 15: Monopolistic competition and oligopoly

Two telecom firms $1$ and $2$ have paid sunk spectrum fees $k_1>0$ and $k_2>0$ with $k_1>k_2$. They produce an identical service at the same constant average cost $c>0$. If they engage in Bertrand competition, the Nash equilibrium prices $(p_1,p_2)$ satisfy