Your starting point here will be the electron configuration of a neutral cadmium atom.
Cadmium, #"Cd"#, is located in period 5, group 12 of the periodic table and has an atomic number equal to #48#. This means that a neutral cadmium atom will have a total of #48# electrons surrounding its nucleus.
This also tells you that the #"Cd"^(2+)# cation, which has two electrons less than the neutral atom, will have a total of #46# electrons.
So, the electron configuration of a neutral cadmium atom looks like this
#"Cd: " 1s^2 2s^2 2p^6 3s^2 3p^6 4s^2 4p^6 4d^10 5s^2#
Alternatively, you can use the noble gas shorthand notation to write cadmium's electron configuration based on that of krypton, the noble gas that comes before cadmium in the periodic table
#"Cd: " ["Kr"] 4d^10 5s^2#
Now, the #"Cd"^(2+)# cation is formed when cadmium loses the electrons that are highest in energy. As you can see in the electron configuration, the two electrons that occupy the 5s-orbital match this profile.
Therefore, the electron configuration of the #"Cd"^(2+)# cation will be
#"Cd"^(2+): ["Kr"] 4d^10#