To answer this, lets get some basic things clear.
Reduction potential = - Oxidation potential
Oxidation means losing of electrons and reduction means gain of
electrons (OIL RIG)
And finally, Oxidation occurs at anode and reduction occurs at
cathode (CR AO)
Standard oxidation potential of Zn is +0.763V and the standard oxidation of potential of Cd is 0.403V.
Since, the standard oxidation potential of Zn is more than that of Cd, Zn electrode will act as anode (where oxidation takes place) and Cd will act as cathode (where reduction takes place).
Ecell = (Eoxidation)anode - (Eoxidation)cathode
= +0.763 - (0.403)
Standard reduction potential of Cd electrode (-0.403V) is greater than that of Zn electrode (-0.763V) so Cd electrode will act as cathode and Zn electrode will act as anode.
Ecell = (Ereduction)cathode - (Ereduction)anode
= -0.403 - (-0.763)
And one more important thing, If only electrode potential is mentioned then that electrode potential is to be assumed the reduction potential by default (not the oxidation potential).