Group velocity of a wave

[ReidMerrill]

Homework Statement

The group velocity of a wave is given by the equation
V_g = dv/d(1/λ)
Prove that the group velocity of a de Broglie particle wave is equal to the ordinary velocity of the particle. Assume the potential energy is equal to zero

Homework Equations

v=h/λ
λ=h/mv

The Attempt at a Solution

I don't know how to start this. None of this was covered in lecture. Any help would be greatly appreciated