The side length AB is √(3²+4²) = 5.
Let's split up the shape in a square and a triangle.
If you introduce point E at (5,1) you see a square ABED with area 5*5=25.
What remains is a triangle with straight sides 5 and √((11-5)² + (1--7)²) = 10. So the triangle's area is 5*10/2 = 25.
This means the total area is 25+25=50.