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You can calculate the area of the triangle knowing its sides.
 For this, we use Heron's formula:
 Area = root ((s) * (s-a) * (s-b) * (s-c))
 s = (a + b + c) / 2
 Where,
 a, b, c: sides of the triangle
 s: semi-perimeter
 The sides of the triangle are:
 For AB:
 AB = root ((- 1 - (- 7)) ^ 2 + (5 - (- 1)) ^ 2)
 AB = 8.485281374
 For AC:
 AC = root ((0 - (- 7)) ^ 2 + (0 - (- 1)) ^ 2)
 AC = 7.071067812
 For BC:
 BC = root ((0 - (- 1)) ^ 2 + (0-5) ^ 2)
 BC = 5.099019514
 Using the formulas we have:
 s = (8.485281374 + 7.071067812 + 5.099019514) / 2
 s = 10.32768435
 Area = root ((s) * (s-a) * (s-b) * (s-c))
 Area = root ((10.32768435) * (10.32768435-8.485281374) * (10.32768435-7.071067812) * (10.32768435-5.099019514))
 Area = 18
 
 the area of triangle ABC is:
 Area = 18