First, you have to know what reacts with Aluminum Nitrate so that you can balance the equation and find out the stoichiometric coefficient. I found a similar question from another website as shown in the picture attached. Balancing the reaction would result to:
2Al(NO₃)₃ + 3FeCl₂ --> 3Fe(NO₃)₂ + 2AlCl₃
The coefficient is 2.