There is 1 Pb and Cr atom on both sides. There are 3 Cl atoms on the left side and 4 on the right side. There are 4 I atoms on the left side and 3 on the right side.
PbI₄ + 4CrCl₃ --> 3PbCl₄ + CrI₃ Cl atoms are now balanced
3PbI₄ + 4CrCl₃ --> 3PbCl₄ + 4CrI₃ All of the atoms are now balanced.