The perimeter 84 is the sum of all 4 sides.  Since a rhombus has 4 equal sides,
each side is  of 84 or 21 each:  



Draw the altitude (in green), label it h:



The area of a parallelogram (and a rhombus IS a parallelogram) is

Area = base×altitude

The base is the bottom side or 21, the altitude is h and the area is 252.

 252 = 21×h
 
  = h
  12 = h

So the altitude is 12.

Edwin