First let's put the words into a reaction:
Silver Acetate + Potassium iodide => potassium nitrate + lead iodide
Now, lets write out these compounds:
AgC2H3O2 + KI => KNO3 + PbI2 ( Now it's easier to see that it is virtually impossible for this to even be a reaction so since you cannot get Ag from Pb. Therefore you can't balance this equation).
So to help you understand, I'll give you a similar example.
Cu + AgNO3 => Cu(NO3)2 + Ag
Now let's analyze how many of each we have on the left side
Ag - 1
NO3(for the most part treat big ions like this, No3, SO4, ClO4,etc, as if they are one element) -1
Now the products side
Compare both sides. Now do you see how there are 2 NO3s on the products side? To balance the equation I would have to have 2 NO3s on the reaction side. Therefore I would add a 2 in front of the Ag. So the equation should look something like this:
Cu + 2AgNO3 => Cu(NO3)2 + Ag
Now do you notice something different? By adding the 2 in front of AgNO3 I also increased the number of Ag, so I now need to add a 2 in front of the Ag on the products side in order to completely balance the equation. The final equation should be:
Cu + 2AgNO3 => Cu(NO3)2 + 2Ag
Compare and make sure each side has the same number of elements.