First identify which is being oxidized and reduced. In this case, the Mg is being oxidized and the Hg is being reduced.
Mg --> Mg+2
<span>Hg+2 --> Hg+1
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Then you have to balance each half reaction first with electrons before adding them together in one equation

⇒

and

⇒
and then combine them together to form

⇒

It isn't necessary to keep the electrons but its essential to know how many there are in order to know how many are in the equation in order to calculate the reaction energy. Note: A<span>dd H+ and H2O to balance the H's and O's in acidic solution if needed.</span>
PbCl₂(s) ⇄ Pb²⁺(aq) + 2Cl⁻(aq)
Pb(CH₃COO)₂(s) → Pb²⁺(aq) + 2CH₃COO⁻(aq)
At lead acetate disolution, concentration of lead cations will increase. According to Le Chatelie's principle equilibrium will be displaced towards formation of solid lead chloride.
The answer is (3). To form the cation (positive ion), Cd must lose negative charge, so two electrons are lost. This loss in electrons also increases the effective nuclear charge/ reduces electron shielding. In other words, with two less electrons to hold, the nucleus exerts a stronger attractive force on the remaining electrons, so the electron cloud is pulled closer to the nucleus. The radius therefore decreases.