This reply may be too late for your assignment/test, but I thought I'd answer anyway.
First, I'm thinking those "slope" values you posted are actually raw absorbance values, from which you need to determine the slope. This slope will have units "change in absorbance per minute". You can calculate the slope by making a simple linear regression plot in Excel.
Your statement of Beer's law is correct, you can just rearrange it a bit. I'm going to use "dA" to mean "change in absorbance. dC is concentration, which you are trying to calculate.
dA/min = dc/min * pathlength * extinction coefficient
Rearrange : dc/min = dA/min* 1/pathlength * 1/extinction coefficient.
Plug in: dC/min = (slope you measured, units 1/min) *1 /(1cm) * 1/ (15,000 M^-1 cm^-1)
at this point the units of cm cancel (one appears in the top, one the bottom). And on the right side the units that remain are M per min. Or mol / (L*min).
You are left with dC/min = (slope you measured) / 15000. Again, units on both sides are M per min.
To calculate the specific activity, use the molecular weight you are give.
I hope this helps, others please feel free to add to or correct me!