Calculus differentiate Discharging circuit question

A cicuit has a capacitor and resistor connected in series with a DC supply.
capacitor = 100 nF
Resistor = 22 kilo ohms
supply= 5V
calculate and estimate the rate of change of voltage when t=T as the circuit discharges.

t is the time in milliseconds
T is the time constant (I've calculated to be 2.2 milliseconds)
I think I need to differentiate but am stuck.