Answer
other diagonal = $2\sqrt 63$
Work Step by Step
As we know the diagonals of a rhombus equally divides each other at 90°.
So one side is the hypotenuse of the right triangle formed by the diagonals of the rhombus.
now let other diagonal is 2x. then the sides of the right triangle is x and 9.
by Pythagoras theorem
$12^{2}=9^{2}+x^{2}$
$144=81+x^{2}$
$x^{2}=63$
$x=\sqrt 63$
other diagonal = $2\sqrt 63$
