Question 793756
Tiling the border of such a pond would require cutting a lot of those tiles/slabs into pieces along slanted lines. A lot of waste in the form of small tile pieces would be expected.
 
Either it is supposed to be a difficult problem; or you are expected to assume that you can cut and put together pieces without wasting any material; or you are just expected to calculate based on the perimeter, not caring about the corners.
 
To figure out the total surface area of the border:
If you are designing a border of uniform width, draw your pond, and then draw lines parallel to the pond borders at the needed distance.
Those lines will form a larger, similar triangle.
Calculate the area of the larger triangle and the pond triangle.
Then, calculate the difference.