We are asked to determine the area of a triangle given its three sides. To do that we will use the following formula:
![A=\sqrt[]{s(s-a)(s-b)(s-c)}](https://img.qammunity.org/2023/formulas/mathematics/college/t3rmhsk71nltcmgtu237bvtgrmrx89v4xv.png)
Where "a", "b" and "c" are the sides of the triangle and the parameter "s" is given by:

Now we find the value of "s":

Solving the operations:

Now we replace the values in the formula for the area:
![A=\sqrt[]{23(23-15)(23-13)(23-18)}](https://img.qammunity.org/2023/formulas/mathematics/college/p2rdedpasf36bdt6e0th4rwldyfsk8ya5f.png)
Solving the parenthesis:
![A=\sqrt[]{23(8)(10)(5)}](https://img.qammunity.org/2023/formulas/mathematics/college/9uwc1o07vzx5mcpum9o5uves65lytuldxh.png)
Solving the product:
![A=\sqrt[]{9200}](https://img.qammunity.org/2023/formulas/mathematics/college/gyr7n3ywm6xtghg6yrjra2o6dpvbgks7us.png)
Solving the square root:

Therefore, the area is 96.0 square kilometers.