Answer:
The answer would be : 7,500 <-- (rounded)
Step-by-step explanation: How i got this answer is, i just multiplied all of the sides together since thats what you do with area's. ( 21 x 21 x 17= 7,497)
Now, the last step is to just round the number off. As we see, the "497" is much more closer to the number "500". So we can just simply round "497" to "500". So your final number for the area would be (7,500) ft.
~hope this helped you out in any way, have a gr8 day/night my friend!~