Answer
$27\ cm$
Work Step by Step
The area of a rectangle with sides a and b is given by:
$$A=ab$$
Since the two rectangles have the same area, we have:
$$
a_1b_1=a_2b_2\Longrightarrow 12\times 18=8\times x\\
\Longrightarrow x=\frac{ 12\times 18 }{8}=27\ cm
.$$
