Answer: Hello there!
First, the area of the earth is:
510.1 million km^2
and the total population of the heart, averaged, is:
7700 million
now we need to see the quotient between these numbers:
A = (510.1 million km^2)/(7700 millions) = (510/7700)km^2 = 0.066km^2
this means that each person gets 0.066km^2
Now, this is not part of the problem per se, but you also need to remember that most of the total area of the earth is inaccessible (the ocean, both poles, etc) where this inhabitable section is almost the 57% of the total area of the earth.
If you want to only divide the habitable area; we get:
A = (510.1*(1 - 0.57))/7700)km^2 = (510.1*0.43/7700)km^2 = 0.066*0.43 km^2 = 0.028 km^2
you can see that if we only divide the habitable area, the portion that you get is a lot smaller.