[SOLVED] Find the rate of change of F with respect to V

The frequency of F of a siren heard by a stationary observer is given by;

F= 132,400/[331 (+-) V], where (+-) V represents the velocity of the accelerating fire truck. Find the rate of change of F with respect to V when,

a.) the fire truck is approaching at a velocity of 30m/s (use -V) and when

b.) the fire truck is moving away at a velocity of 30m/s (use + V)

Well what i did was first differentiate 132,400/[331 (+-) V], substituting the numbers with variables. Let a= 132,400 and b= 331; my answer was

-a/ [b (+-) V)^2]. From that I worked out number a where i used -V and for number b i used +V. My answers of that I don't know what to do with them... I don't know it is just that, if they are correct that is, or i need to do more. For a.) i got -1.46 and the other -1.02..