You dont find it with the law of cosine....
you use the law of cosine to find the length of the triangle's edge. if you want to find the triangle's area you need to use the law:
It depends upon what we know about the triangle......
If we know two sides and an included angle between these sides, then Erlich's approach is correct.
If we only know the three sides, we can use someting known as "Heron's Formula" which is
Area = sqrt [ s * (s - a) * (s - b) * (s - c) ]
Where a, b, c are the sides and "s" is the semi-perimeter given by
[ a + b + c ] / 2