As an example, let's use a triangle with side lengths 4, 5, and 7. The first step is the compute Z = (4 + 5 + 7)/2 = 8
The next step in finding the area of a triangle is to compute the product
(Z)(Z-4)(Z-5)(Z-7) = (8)(4)(3)(1) = 96
The last step is to take the square root of the number you obtained in step two. Sqrt(96) = 9.8, so the area of the triangle is 9.8.
More generally, if the side lengths of the triangle are A, B, and C, then the area is Sqrt[Z(Z-A)(Z-B)(Z-C)], where Z = (A + B + C)/2.