In a neutralization, you have a substance that is classified as an acid (H3PO4) being combined with a substance that is classified as a base (Ca(OH)2). Acids are substances that dissociate into hydrogen ions (H+), while bases are substances that dissociate into hydroxide ions (OH-). The combination of these two during a neutalization produces a salt (Ca3(PO4)2) and water (H2O.
Here is the unbalanced equation:
H3PO4 + Ca(OH)2 ---> Ca3(PO4)2 + H2O
I teach my students to set up a two-column table to list the atoms involved in the reactant side of the equation and the atoms involved in the product side of the equation.
H 5 2
P 1 2
O 6 9
Ca 1 3
With the use of coefficients, which are big numbers in front of the molecules you want to change the number of, you can manipulate the number of atoms on both sides of the yields sign. Don't forget to change the numbers in the table as you do this. I would start with trying to fix the calcium on the reactants side, by putting a 3 in front of it. Then, I would put a 2 in front of the H3PO4. The final accounting touch would be to place a 6 in front of the H2O on the products side to give the balanced equation:
2H3PO4 + 3Ca(OH)2 ---> Ca3(PO4)2 + 6H2O