What type of reaction is this? How do you balance it? #Hg(OH)_2 + H_3PO_4 -> Hg(PO_4)_2 + H_2O#
It's a neutralization reaction.
This is a neutralization reaction between a strong acid and a strong base.
First off, you missed a subscript of 3, in mercury (II) phosphate; should be
To balance it, you have to consider the subscripts and use coefficients effectively, so there is an equal amount of elements on either side of the equal sign.
Hope this helps :)