Since you already know the product of j and k, all you need to know is the value of L. If you know the dimensions are all integers, then j and k must equal 7 and 3 because they are the only integers greater than 1 whose product is 21. You know that one of those values (7 or 3) multiplied by L = 30. Again, because they are all integers, it would have to be 3 because it goes into 30 evenly:
3 (L ) =30
L = 10
Now you can multiply the base area (21) and the height (10):
210