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Go to the following link which will give you a better understanding of finding the area of a triangle knowing 3 sides.
Click on the following link: http://www.mathopenref.com/
Follow the following path:
Plane Geometry > Triangles > Triangle Perimeter and Area > Area using Heron's formula
Follow the following path:
Plane Geometry > Triangles > Triangle Perimeter and Area > Area using Heron's formula