The velocity of the source is positive if the source is note that this equation may not use the sign convention you are accustomed to. think about the physical situation before answering.

source is travelling away from the listener.

Explanation:

we know that

v= speed of sound

v_L= speed of listener

v_s=speed of the source

Let us consider the case when vL is zero.

We can see from equation that if we put +ve value for vs , then fL turns out to be less than fs.

(As denominator becomes greater than numerator part in right side term)

Frequency as heard by listener is less than the frequency of source when source is moving away from listener.

Therefore in this equation velocity of source is positive if the source is travelling away from the listener.

the closer the planet is to the sun, the larger the gravity