Do you want to put the Contents into the array at the rack# position or to you want and array filled with (rack#,contents)?
If you want to store contents AT the rack# position then you can use many many ways to enter elements into the array one such is
MOV(contents,rack_and_contents[rack#])
otherwise you should make a 2d array rack_and_Conents[x,2]
and put rack# in at rack_contents[x,0]
and contents in at rack_and_contents[x,1]
As for what specific instruction. As long as destination field is the element in the array you want the result will get stored there