The combined area of Canada and the United States covers about what percent of the world's land area

13%

Explanation:

Russia is the largest country in the world, followed by Canada, and then the United States. Both Canada and the United States make up North America. The landmass occupied by a country determines if it is large or small. Earth has a landmass of 510,072,000 km^2. Of this landmass, Canada occupies 9,984,670 km^2. This is 6.7% of the earth's landmass. The United States occupies 9,833,516 km^2. This is 6.5% of the earth's landmass. The addition of 6.7% and 6.5% gives us approximately 13%.

So, the two countries occupy 13% of the earth's land area. Russia, which happens to be the largest country of the word, has a land area of 17,098,242 km^2, and this alone represents 11.5% of the earth's land area.