Related rates:
M=Msun−2.5log10(L/Lsun). where Msun=4.75 and Lsun=3.839×1026watts. (Note that, by convention, brighter stars have lower magnitude.) If the absolute magnitude of a variable star is decreasing at a rate of 0.09/week, how quickly is the luminosity of the star increasing when the magnitude is 3.8?

I am having trouble with the derivative, and I feel like I'd be missing values to solve this problem if I did take the derivative.

Question

Related rates:
 M=Msun−2.5log10(L/Lsun). where Msun=4.75 and Lsun=3.839×1026watts. (Note that, by convention, brighter stars have lower magnitude.) If the absolute magnitude of a variable star is decreasing at a rate of 0.09/week, how quickly is the luminosity of the star increasing when the magnitude is 3.8? 

I am having trouble with the derivative, and I feel like I'd be missing values to solve this problem if I did take the derivative.