How to Determine the Sign of the Doppler Shift?

[nao113]

Homework Statement

If the velocity of the target is positive, ∆𝑓 > 0.
If the velocity of the target is negative, ∆𝑓 < 0. How to identify the sign of ∆𝑓 in Doppler system?

Did I answer it correctly? thank you

Relevant Equations

I put that in the picture below

answer:

 

[haruspex]

I don't know what answer is wanted. As I read it, both the problem statement and your answer assert that the sign of the shift tells you whether the relative velocity is positive or negative. But the question is asking how you determine the sign of the shift, no?