Help with 2nd derivative in physics problem please.

I am trying to follow a derivation and am stuck with this step.

If p=r/a, then du/dr = (1/a).du/dp (which i think I'm OK with)

and d2u/dr2 = (1/a)^2.d2u/dp2 (which I'm not)

(2's are powers here of course)

I'm not sure if it is relevant but earlier on in the text a substiitutuion
is used
u = r.psi(r) where psi is a function of r

Thanks
Zinc

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