Fill in the coefficients that will balance the following reaction:
+ → +

Our coefficients should be:

NaCl: 2
CaS: 1
Na2S: 1
CaCl2: 1

Why? 
 NaCl and CaS are our reactants. So, in other words, with these coefficients, we have 2 Na's, 2 Cl's, 1 Ca, and 1 S. To obey the law of conservation of mass, we need to make sure that we have an equal number of each element on the PRODUCTS side as well. So, if we have 2 Na's for example, we need to have 2 on the other side as well.

So, if we use the coefficient of 1 for Na2S, that tells us we have 1 S and 2 Na's. If we look back on the other side,we have 2 Na's and 1 S. So, this part is balanced and equal on both sides.

If we use the coefficient of 1 for CaCl2, we have 1 Ca and 2 Cl's.
If we look back on the other side, after the coefficients have been placed, we have 1 Ca and 2 Cl's on the reactants side as well.

So, now that each side of the equation has the same number of each element, it is balanced and equal.

Please let me know if you have any questions, I hope I could help!