Explanation

(b)
The given equation of rectangular hyperbola is $xy = 18$ …(i)
On comparing Eq.(i), with general equation of rectangular hyperbola
$xy = \frac{a^{2}}{2}$
We get, $\frac{a^{2}}{2} = 18\ \Rightarrow a^{2} = 36$
$\Rightarrow a = 6$
$\therefore\$Length of the transverse axis of rectangular hyperbola is $2a = 2 \times 6 = 12$