Call the length of the smaller rectangle $x$.

If the area of the smaller rectangle is $2x$, the area of the larger rectangle is $4x$, and the height of the larger rectangle is $4x/6$. The height:length ratio of both rectangles must be the same, since they are similar.

So $\frac{2}{x} = \frac{4x}{36}$

$x^{2} = 18$

$x=\sqrt{18}$

Alternatively, call the height of the larger rectangle $y$.

Then, comparing the areas, $4x = 6y$

By similarity, $\frac{y}{2} = \frac{6}{x}$

i.e. $y = \frac{12}{x}$

Then by substitution, $4x = \frac{72}{x}$

$x^{2} = 18$

$x = \sqrt{18}$