THE FEDERAL DEBT D IN TRILLIONS OF DOLLARS HELD BY FOREIGN AND INTERNATIONAL INVESTORS x YEARS AFTER 1970 CAN BE MODELED BY

D(x) =
WHERE 15 ≤x≤ 40.

The given equation is incomplete, and it seems that some information is missing after "D(x) =". However, I can help you understand the equation and how to interpret it if you can provide the missing information.