It’s a reaction.
##3 Hg(OH)_”2″ + 2 H_”3″PO_”4″ -> Hg_”3″(PO_”4″)_”2″ + 6 H_”2″O##
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 ##Hg_”3″(PO_”4″)_”2″##
To balance it, you have to consider the subscripts and use coefficients effectively, so there is an equal amount of on either side of the equal sign.
##3 Hg(OH)_”2″ + 2 H_”3″PO_”4″ -> Hg_”3″(PO_”4″)_”2″ + 6 H_”2″O##
Hope this helps 🙂