The area of an equilateral triangle is $400\sqrt{3}\text{sq. m}$. Its perimeter is:

The area ($A$) of an equilateral triangle with side length $a$ is given by:
$$A=\frac{\sqrt{3}}{4}{a}^2$$

Given:

$$\frac{\sqrt{3}}{4}{a}^2=400\sqrt{3}$$

$$a^2=400\times 4=1600$$

$$a=\sqrt{1600}=40\text{m}$$

The perimeter of an equilateral triangle is:

$$\text{Perimeter}=3a=3\times 40=120\text{m}$$