The problem is stated clearly on this poster from Nrich:
(All Nrich posters)
Check the problem on Nrich and also the Teachers’ resources.
This activity could be perhaps started on paper or on a Geoboard and once students have tried a few cases and can find the areas themselves go on to explore the problem further with technology which will allow many more cases to be considered.
Try Pick’s Theorem Explorer from Steve Phelps or Pick’s Formula – Alex Chik