mass of water * 1 cal /g °C * (Equilibrium temperature - initial temperature of the environment) = mass of the environment * Cs of the environment * ( Initial temperature of the environment - Equilibrium temperature)
From that you can solve for the Equilibrium temperature.
If it is not a closed system, both the jars are in contact with an indefinite large amount of air, then the jars will reach the temperature of the air that surrounds the jars and that will be the thermal equilibrium.
