At first you need to balance ca's and p's.
Then we will have : [tex]3 Ca(OH)2 + 2 H3PO4 -\ \textgreater \ Ca3(PO4)2 + 6 H2O[/tex]
And that's all you need to give the answer.
If you're wondering how I did it, here are my steps :
3H3PO4 + Ca(OH)2 = Ca(H2PO4)2 + H2O
2H3PO4 + 2Ca(OH)2 = Ca2(HPO4)2 + H2O
2H3PO4 + 3Ca(OH)2 = Ca3(PO4)2 + 6H2O
