Chemical Kinetics

For a zero order reaction $\mathrm{A} \longrightarrow$ product. Conc. of $A$ decreases from $0.8 \mathrm{~mol} \mathrm{~dm}^{-3}$ to $0.2 \mathrm{~mol} \mathrm{~dm}^{-3}$ in 6 minute. What is rate constant of the reaction?