To go from the statement D -> Not A to the statement A -> Not D we are taking the contrapositive. A contrapositive is an alternative (but equivalent) form of any conditional wherein we swap the places of the terms and negate both sides. So flip the positions of D and Not A, and then negate both sides, and we get A -> Not D. If you would like more information on contrapositives feel free to check out the sufficient and necessary lesson in the course!
