Electrochemistry: explain question

A student wants to measure the standard electrode potential of the $\ce{Fe^{3+}/Fe^{2+}}$ half-cell. The student sets up a cell with a platinum electrode dipped into a solution containing both $\ce{Fe^{3+}(aq)}$ and $\ce{Fe^{2+}(aq)}$ ions, each at a concentration of 1 mol dm$^{-3}$, and connects this half-cell to a standard hydrogen electrode. The student records a cell potential of +0.77 V.

Explain why a platinum electrode is used in the $\ce{Fe^{3+}/Fe^{2+}}$ half-cell, what the +0.77 V reading represents, and how the student knows the polarity of the platinum electrode in this cell.