Answer
The unit for the rate constant is $M^{-2} s^{-1}$
Work Step by Step
Since the units at both sides of the equation must be equivalent:
$M \times s^{-1}=x\times M^2 \times M$ (with $x$ is the unit for $k$)
$M \times s^{-1}=x\times M^3$
$x=\frac{M \times s^{-1}}{M^3}$
$x=M^{-2} s^{-1}$
