How do you solve the d(λmu) equation in relativistic energy?

[kbeach]

Homework Statement

I am trying to grasp this stuff, but it is getting late and my mind is noodeling. Can I have some help on this?
show that d(λmu) = m(1-(u^2/c^2)^(-3/2) du
thanks!

Homework Equations

λ = 1/√(1-(u^2/c^2))

The Attempt at a Solution

I have attacked this through integration and differentiation, but am still missing something

 

[vela]

Show your work.