the diagonals of a rhombus are and
the diagonals are all crossed at one point...at this point the lengths are perfectly split in half resulting in the short diagonal becoming only 12cm and the to become
the diagonal lines of the perimeter by using the Pythagoras' theorem:
...-> being the unknown (one of the sides or the hypotenuse that is diagonal)
Now you've got one of the sides solved, multiply it by since has sides.