First write down the chemical formula:
Fe(NO3)3 + K2S
Then, to determine the products use the AD+CB method to form your products:
A = Fe
B = (NO3)3
C = S
D = K2
Products: Fe2S3 + KNO3
Whole equation:
Fe(NO3)3 + K2S -> Fe2S3 + KNO3
Write the balanced chemical equation:
2 Fe(NO3)3 + 3 K2S -> Fe2S3 + 6 KNO3