Displacement, velocity and acceleration

[pookisantoki]

A Whale Swims due east for a distance of 3.71km, turns around and goes due west for 3.29km, and finally turns around again and heads 7.69km due east. What is the magnitude of the displacement of the whale?

I tries to solve it using the x=xfinal-x initial
3.29-3.71=-0.42
-0.42-7.29=-8.11km
and my final answer was -8.11km but it's wrong...What am i doing wrong?

 

[Redbelly98]

Perhaps they want the answer given as magnitude and direction, i.e. "east" or "west".

Or perhaps they want you to use east as the positive direction, not negative as you did.