Answer
54
Work Step by Step
In order to use Heron's Formula we have to find the semiperimeter:
$s=\frac{1}{2}(a+b+c)$
$s=\frac{1}{2}(9+12+15)$
$s=\frac{1}{2}*36$
$s=18$
We can use Heron's Formula from here:
$A=\sqrt{s(s-a)(s-b)(s-c)}$
$A=\sqrt{18(18-9)(18-12)(18-15)}$
$A=\sqrt{18*9*6*3}$
$A=\sqrt{2916}$
$A=54$
