Sometimes it helps to draw it out...
(17)
C_______M_________Q
|___________________|
21
therefore, CM + MQ = CQ
17 + MQ = 21
MQ = 21 - 17
MQ = 4 <===
The initial measure is:
m1 = 28 ft
The real measure must be:
m2 = 14 ft
The correct procedure to determine the scale factor is:

Dividing both numbers between 14 we have:

Rewriting:
Answer:
Zelie should have divided both numbers by 14.
The answer is 6.524. Hope this helps! :D