Answer
$\approx62.78$ square miles
Work Step by Step
The area of a triangle with sides $a,b,c$ is: $\sqrt{s(s-a)(s-b)(s-c)}$ where $s=0.5(a+b+c)$.
Hence here $s=0.5(18+8+23)=24.5$, thus the area: $\sqrt{24.5(24.5-18)(24.5-8)(24.5-23)}\approx62.78$ square miles
