In order to calculate the area of the triangle, given the length of its three sides, we can use Heron's formula:
[tex]A=\sqrt{p\left(p-a\right)\left(p-b\right)\left(p-c\right)}[/tex]
Where p is the semi-perimeter.
So, calculating the value of p and then the area of the triangle, we have:
[tex]\begin{gathered} p=\frac{a+b+c}{2}=\frac{18+20+22}{2}=\frac{60}{2}=30 \\ A=\sqrt{30\left(12\right)\left(10\right)\left(8\right)} \\ A=\sqrt{28800} \\ A=169.7\text{ ft^^b2} \end{gathered}[/tex]
Rounding to the nearest square foot, the area is 170 ft².