The final temperature is 28 °C.
There are two heat transfers involved.
heat from combustion of propane + heat gained by water = 0
q_1 + q_2 = 0
q_1+ C_calΔT = 0
q_1 = -50 kJ
q_2 = 6.00 kJ·°C^(-1) × ΔT = 6.00 ΔT kJ·°C^(-1)
q_1 + q_2 = -50 kJ + 6.00 ΔT kJ·°C^(-1) = 0
ΔT = 50/[6.00 °C^(-1)] = 8.33 °C
T_f = T_i + ΔT = 20 °C + 8.33 °C = 28 °C