Q.What is a redox reaction ?

Cd is oxidised (anode) and Cu²⁺ is reduced (cathode); subtracting their standard potentials gives +0.737 V.

In the reaction $Cd(s) + Cu^{2+}(1M) \rightarrow Cd^{2+}(1M) + Cu(s)$, cadmium is oxidised (anode) and copper(II) ion is reduced (cathode). The standard cell potential is:

$E^\circ_{cell} = E^\circ_{cathode} - E^\circ_{anode} = E^\circ_{Cu^{2+}/Cu} - E^\circ_{Cd^{2+}/Cd} = 0.334 - (-0.403) = 0.737\ V$

Since this is positive, the reaction as written is spontaneous.

Subtracting the cell EMF from the silver electrode potential gives the copper electrode's standard potential.

In the cell $Cu(s)\,|\,Cu^{2+}(1M)\,||\,Ag^+(1M)\,|\,Ag(s)$, copper (written on the left) is the anode (oxidation) and silver is the cathode (reduction):

$E^\circ_{cell} = E^\circ_{cathode} - E^\circ_{anode} = E^\circ_{Ag^+/Ag} - E^\circ_{Cu^{2+}/Cu}$

$0.463 = 0.800 - E^\circ_{Cu^{2+}/Cu}$

$E^\circ_{Cu^{2+}/Cu} = 0.800 - 0.463 = 0.337\ V$