Find Group Velocity: Solving for k=5.8x10^9 m-1

[magnifik]

given: E(k) = 2.5 x 10^-28 k² Joules
k = 5.8 x 10⁹ m^-1

Find the group velocity.

i know that the equation for v_g = 2pi/h (dE/dk)
dE/dk = 2(2.5 x 10^-28) = 5.0 x 10^-28 k
plugging in k, i get
dE/dk = 2.9 x 10^-18 J/m
v_g = 2.75 x 10¹⁶ m^-1 s^-1

what i am more concerned about here is the fact that my units should be m/s (or at least i think so), not 1/m-s. can anyone tell me why I'm getting the wrong units?

 

[Point Conception]

Have you seen this page:
http://www.mathpages.com/home/kmath210/kmath210.htm

 

[magnifik]

Have you seen this page:
http://www.mathpages.com/home/kmath210/kmath210.htm

yes, however they use angular frequency as opposed to energy. it does confirm that the units should be in m/s though