Chapter 2 - Atoms, Molecules, and Ions - Exercises - Page 77: 2.74d

$Fe(ClO_4)_2$

Again, this is an ionic compound. Since the name tells us that it is Iron (II), we know that the iron ion has a charge of +2. The perchlorate ion is the anion, which has a formula of $ClO_4$ and a charge of -1. To get a neutrally charged compound, there needs to be twice as many molecules of perchlorate as atoms of iron, hence the $Fe(ClO_4)_2$.