The correct answer to the problem is that it is impossible to determine. The fact that it was travelling at 586 mph at moment of impact doesn't mean that was its speed during the entire flight.
It's a high school algebra story problem, not a college-level physics problem. It's solvable using the level of problem solving tools available to a high school student. That said, it could have been an airplane named "A" and a crash site named "B." Wasn't kind using the 9/11 circumstances, especially in Massachusetts.
If I was a teacher grading the problem as I described it, without the 9/11 emotionalism, and a student gave me the answer you gave me, I'd have chuckled as I drew a big red "Zero" on your paper.