To calculate for the area of the triangle given all its side, use the Heron's formula which is,
A = sqrt ((s)(s - a)(s - b)(s - c))
where A is the area. a, b, and c are the measure of the sides and s the semi - perimeter or half of the perimeter.
With the given above, s = (17 + 18 + 21.4) / 2 = 28.2
Substituting the known dimensions to the Heron's formula,
A = sqrt ((28.2)(28.2 - 17)(28.2 - 18)(28.2 - 21.4)) = 148 units^3
Thus, the area of the triangle is 148 units^3.