Since you need everything in the same unit of measurement (I used m and m^2)

12km = 12,000m

1hectare = 10,000m^2

From this you need to find out the total area of the farm and then multiply by 8 to get the number of people

The people will be 288 hectares x 8 people/hectare = 2304 people

Dimensional Analysis is very handy for this kind of problem, it helped me understand :]