gas flow rate :- 40 cubic meter per hour at 1.35 kpa

Gas:- Natural gas(Methane:- 100 percent)

Air flow:- 400 cubic meter per hour at 17 kpa

Burner type:- High velocity gas burner

After combustion gas flows in a combustion chamber transferring heat to steel wall of length:- 20 metres of the crossection(350mm wide x 2500mm high)

i would like to calculate the speed of the gas of product of combustion

Please let me know the best way to calculate this