The number of liters of water remaining in a tank t minutes after the tank has started to drain is R(t)=2t^3-20t^2-72t+820. At what moment is the water draining the fastest?

the first thing i think of is to take the derivative but then after that i dont know what to do.

i took 2nd derivative then set it to 0

then i get

x= 10/3