Describe a situation in which the net external force on a system is not zero, yet its speed remains constant.

the entire statement is true and correct.  

there's no diagram, nothing is illustrated, and no question is asked.

the total distance that martina travels that day is

(distance from home to mom's office) plus (distance from mom's office to school)

plus (distance from school to library) plus (distance from library to friend's house)

plus (distance from friend's house to martina's house).

(we don't know what any of these distance actually is.)

answer; how far out of place which the object finished relative to the starting location;