Find a value in a matrix

Hi all, I have poltted on spectrum (x=Raman shift, Y= intesity) and I'd like to get the position of one peak in the matrix i.e.find the index of the the peak at 1400.

thanks for your help

Answer 1

[John D'Errico]

help max

Look at the SECOND argument returned from max.

[Jacopo Zini]

Thanks, but I did not get it