15 cm if you satisfied please mark as best
Draw a rhombus and name it abcd and then draw diagonals and take any of them with 18 cm and 24 cm i have taken DB as 24 and AC as 18. Now, there is a property of rhombus that its diagonal bisects at 90 degree and they bisect each other i.e they are divided equally then we can see that DC or any side of rhombus is the hypotenuse therefore (12)2 + (9)2 = (DC)2
144 + 81 = DC2
225= (DC)2
root of 225 = Dc 
15 is the root of dc 
Hence 15= DC