Hey-- let me try to explain.
To understand this concept better, look at question #52. Look at silicon's structure. It has 4 valence electrons, so those can be removed easily. It would be very difficult to remove the 5th electron because the electronic structure is very stable now-- because the subshells are filled. So getting back to the idea that 4 electrons were originally removed easily--- but once they were removed it is very stable- and as a result the 5th ionization energy is very large.
Now, back to question #45. First draw the electron configuration out. Mg has 2 valence electrons, so those will be removed easily. As a result, the third ionization energy would be large compared to that of the 2nd. Remember- ionization energy is the energy needed to remove one mole of electron. It would be difficult to remove the electron after the valence electrons are gone, therefore- the ionization energy is large.
I hope that was helpful!
