I hope you know basic calculus. I am answering your question with it.
Let the volume and a side of a cube be V m^{3} & a m. respectively.
So, V=a^{3}
Now differentiate both sides with respect to time t.
i.e. dV/dt = 3a^{2}da/dt
We have to find da/dt while a=6 & we know dV/dt=2.
i.e. 2 = 3 x 6^{2} x da/dt.
or, da/dt = 2/(3 x 36) = 1/54 m/s.