You could choose the following points:
A(1, 0), B(11, 0) and C(1, 10)
Then length AB is 11-1 = 10; length AC is 10-0 = 10, and length AC is found from the Pythagorean theorem to be ...
... √(10²+10²) = √200 = 10√2
The perimeter fence will have length 10 + 10 + 10√2 ≈ 34.14 units