Which coefficient is needed in front of nano3 to balance the equation na2s + zn(no3)2 → zns + _nano3?

To balance the equation, we need to write how many elements is on each side of the equation. 

Since the coefficient will apply to the whole compound (all the elements) we need to determine how many is on the reactants side. 

Na=2 
O=6
N=2   

Since their is already a subscript of 3, we can already conclude that we multiply 2*3=6. Therefore, our answer would be 2.

To balance the equation, we need to place a coefficient of 2 in front of NaNO3.