What is the balanced equation of

H3PO4+CaH2O = Ca3(PO4)2+H20 ?

assign a variable for each one

a = H3PO4
b = CaH2O
c = Ca3(PO4)2
d = H20

write an equation using these values, for each element...

H: 3a + 2b = 2d
P: a = 2c
O: 4a + b = 8c + d
Ca: b = 3c

solve.

2(H3PO4)+ 3(Ca(OH)2)= Ca3(PO4)2 + 6 (H2O)