How can I use Dalton's Law to solve this problem?

Gee Aaron, unless I'm missing something, this is really easy. The total pressure is 687 torr, and the pressure of the water vapor is 21.2 torr.Well, the total pressure is just the sum of the partial pressures of each of the ges. You only have two, so the pressure of the CO2 is just 687.0 torr - 21.2 torr = 665.8 torr.