Answer:
Density is nearly 3 cabs per hectare
Step-by-step explanation:
Given - 
There are three three intertidal regions. 
Area of each intertidal region is 
 hectares.
Number of crab in the intertidal regions are as follows - 
Intertidal region I - 7
Intertidal region I - 2
Intertidal region I - 6
Sum of crabs in all three regions is equal to 

Total area of all three intertidal regions is equal to 
 hectares
Now density per hectare is equal to total population of crab divided by total area of all the three intertidal regions 
Density 

Density is nearly 3 cabs per hectare