Answer
$17$ square kilometers.
Work Step by Step
Using that we have obtained the lengths of the sides: $s=0.5(a+b+c)=0.5(5+7+9)=10.5$
Then the area of the triangle is: $\sqrt{s(s-a)(s-b)(s-c)}=\sqrt{10.5(10.5-5)(10.5-7)(10.5-9)}\approx17$ square kilometers.