Work Step by Step
First, look at the charges of the ions in Zinc Nitrate. The Zinc ion has a charge of 2+. On the other hand, Nitrate $(NO_3)-$, has a charge of 1-. This -1 charge is only half the magnitude of the charge of Zinc, which is 2+. This means that in the chemical formula, there must be two Nitrate ions for every one Zinc ion. Metals come first in the chemical formula, so the formula for Zinc Nitrate is $Zn(NO_3)_2$.