Just trying to verify if my answers were correct

Just divide:

[7.6 x 10^9] / [5.1 x 10^8] =~15 people per km^2 - human population density.

30% x [5.1 x 10^8] =1.53 x 10^8 Km^2 - the surface area of all the land.

[7.6 x 10^9] / [1.53 x  10^8] =~ 50 people per km^2 on all habitable land. 

In the first situation, the population density is given by :

[ 7.6 x 10⁹ ]              7.6

__________  =      ____   x   10   ≈   14.9  people / km^2

[5.1 x 10⁸ ]              5.1

In the second situation, we have

[7.6 x 10⁹ ]                    7.6

______________   = ________  x 10   ≈   49.7  people /km^2

[ 5.1 x 10⁸ x .30]           5.1 x .3