Answer
The formula of phosphoric acid is $H_{3}PO_{4}$.
Work Step by Step
Oxyacids with oxanions ending in -ate are named as : $ 'base name \hspace{1mm}of \hspace{1mm}oxyanion' + ic + acid$
Here, $phosphor$ is the basename and the oxyanion ends in $-ate$. so, the oxyanion is phosphate: $PO_{4}^{3-}$. To balance the charges, 3 hydrogen ions are present. So, the formula of phosphoric acid is $H_{3}PO_{4}$.
