what flatlands extend south from cheapsake bay to the east coast of florida?

That region is often called the Inner Banks. This distinguishes it from the Outer Banks, which are barrier islands.

The flatlands that extend south from Chesapeake Bay to the east coast of Florida are called the Coastal Plain. These low-lying areas are characterized by their relatively flat terrain and proximity to the Atlantic Ocean. To find this answer, one can use various resources, including regional geography textbooks, online sources such as encyclopedias or geography websites, or consult maps that show the different geographic regions of the United States.