pv=c, where p=pressure, v= volume and c=constant.

When the volume is 6 cm^3, the pressure is 2 kg/cm^2 what is the instantaneous rate of change of the volume if the pressure decreases by 200g/cm^2 every minute?

So instantaneous rate of change is the same as the derivative, from what I understand. Usually to find this I would multiply d/dt into the equation, find the derivatives,isolate for the variable I am looking for and sub in the given values to solve. In this instance I just don't know where c fits in. Can anyone help me figure it out?