What is the henry's law constant for co2 at 20∘c? express your answer to three significant figures and include the appropriate units?

Actually, Henry's Law is an empirical value. It means that it was not obtained out of raw calculations or correlations. This was gathered from experimental results. Hence, you can search its data. At standard temperature of 25°C (298 K),

k = k°e^[2400 (1/T - 1/T°) ], where k° = 29.4 L·atm/mol

Substituting the values so that T would be in 20°C or 293 K,

k = (29.4 L·atm/mol) e^[2400 (1/293 - 1/298) ]

k = 33.7 L·atm/mol