Your answer is [tex]P_{2}O_{3}+3H_{2}O\rightarrow\ 2H_{3}PO_{3}[/tex]
but if you are looking for some explanation check description below :)
Diphosphorus trioxide formula is:
[tex]P_{2}O_{3}[/tex]
Di stands from 2 atoms of something, in this case we have 2 atoms of Phosphorus.
Trioxide means that we have 3 atoms of oxygen in this substance that's why we have: [tex]P_{2}O_{3}[/tex]
And water is just [tex]H_{2}O[/tex]
Unbalanced formula is:
[tex]P_{2}O_{3}+H_{2}O\rightarrow\ H_{3}PO_{3}[/tex]
Now we need to balance this equation
On left hand side we have 2 atoms of Phosphorus, on right hand side we have just one atom of Phosphorus, that's why we need to put 2 before [tex]H_{3}PO_{3}[/tex]
Now we need to count all atoms of oxygen and hydrogen and make it equal on both side.
So we need to put 3 on left hand side before water to make this equation balanced.
That's why correct answer is:
[tex]P_{2}O_{3}+3H_{2}O\rightarrow\2H_{3}PO_{3}[/tex]
