Let the length and breadth of the rectangle be $3 x \mathrm{~m}$ and $x \mathrm{~m}$, respectively.

According to the question:

$\begin{array}{l}
3 x \times x=147 \\
\Rightarrow 3 x^{2}=147 \\
\Rightarrow x^{2}=49 \\
\Rightarrow x=7 \text { or } x=-7 \\
\Rightarrow x=7(\because x \text { cannot be negative) } \\
\therefore \text { Length }=3 \times 7=21 m \\
\text { Breath }=7 m
\end{array}$