Question: find the rate of change of a volume of grains in the silo when the volume of grains in a silo at a particular time is given by V(t)=4t(3-t)m^3.
What you are being asked for here is the time-rate-of-change of the quantity V, given by V(t) = 4t(3 - t).
DIfferentiating V wrt time, t,
dV/dt = d/dt(12t - 4t^2) = 12 - 8t
dV/dt = 12 - 8t m^3/min (assuming t is measured in minutes, rather than seconds, say)