Let the volume and a side of a cube be V m3 & a m. respectively.
Now differentiate both sides with respect to time t.
i.e. dV/dt = 3a2da/dt
We have to find da/dt while a=6 & we know dV/dt=2.
i.e. 2 = 3 x 62 x da/dt.
or, da/dt = 2/(3 x 36) = 1/54 m/s.