Option 3 is correct because it has parentheses around 40 - x, but not the cube itself or the 3rd power if you didn't know what I meant. Anyway, the reason this is correct is because if you look at any order of operations, you will notice that parentheses always comes before powered numbers, so if you were to choose, say option 2, it would be a completely different answer than option 3, not to mention wrong also.
