The length of a rectangular park is twice its breadth, and its perimeter measures 840 m.

Solution:

Let the breadth of the rectangular park be $b$.

$\therefore$ Length of the rectangular park $=l=2 b$

Perimeter $=840 \mathrm{~m}$

$\Rightarrow 840=2(l+b)$

$\Rightarrow 840=2(2 b+b)$

$\Rightarrow 840=2(3 b)$

$\Rightarrow 840=6 b$

$\Rightarrow b=140 \mathrm{~m}$

Thus, we have:

$l=2 b$

$=2 \times 140$

$=280 \mathrm{~m}$

Area $=l \times b$

$=280 \times 140$

$=39200 \mathrm{~m}^{2}$