Hello there,
So the first thing that we would want to do it to multiply each number that is shown in the first little cube on top.
First top little cube shows: 5*5*5 and this would equal 125.
The Bottom Big Cube: This show's 15*15*15, and it would equal you: 3375.
So the last thing we would want to do is add the top cube and the lower cube together.
3375
+125
______
3500
Hope this helps. :)