The answer would be "Sixty-one billion, four hundred three million, nine hundred twenty-one thousand, five hundred seventy-five".
You can find this by labeling each section (billions, millions, thousands, etc), and writing out each number. Numbers like 61, 21 and 75 have a hyphen between the tens place and the ones place. Doing this, you'll get "sixty-one billion, four hundred three million, nine hundred twenty-one thousand, five hundred seventy-five" as an answer.
I hope this helps!
