Answer:
8.5 feet.
Step-by-step explanation:
A sketch of the position of the three people gives a right angled triangle. The hypotenuse of the triangle gives the resultant displacement, while the two other sides are 6 feet each.
The resultant displacement, R, is the overall displacement covered by the doctor. This can be determined by;
R =

where:
= 6 feet and
= 6 feet.
Thus,
R =

=

R = 8.49
The resultant displacement covered by the doctor is 8.5 feet.