The amount of energy carried by a sound wave through a given area in a given time is the intensity $I$. It can be calculated via power $P$ and area $A$ as follows:

$$
begin{equation}
I = frac{P}{A}
end{equation}
$$

We know that the power of the sound source is $P = 0.21text{ W}$. To find the area through which it has to pass to have an intensity $I = 0.067 ; frac{text{W}}{text{m}^2}$ is done simply, using formula (1) as follows:

$$
begin{align*}
I = frac{P}{A}
end{align*}
$$

Rearranging for $A$:

$$
begin{align*}
A = frac{P}{I}
end{align*}
$$

Plugging in the values we get:

$$
begin{align*}
A = frac{0.21text{ W}}{0.067 ; frac{text{W}}{text{m}^2}} approx 3.1text{ m}^2
end{align*}
$$