Answer
$$A=\sqrt{6479}$$
Work Step by Step
By Heron's Formula, we know:
$$A=\sqrt{s(s-a)(s-b)(s-c)}$$
Where:
$$s=\frac{a+b+c}{2}$$
Thus, we find:
$$ s=\frac{12+20+30}{2}=31 \\ A=\sqrt{31\left(31-12\right)\left(31-20\right)\left(31-30\right)} \\ A=\sqrt{6479}$$