When the equation for the reaction represented below is balanced and the coefficients are reduced to the lowest whole number terms, the coefficient for ${H_2}O$ is:
$KO{H_{\left( {aq} \right)}} + {H_3}P{O_{4\left( {aq} \right)}}\xrightarrow{{}}{K_3}P{O_{4\left( {aq} \right)}} + {H_2}{O_{\left( l \right)}}$

Hint: We know that a balanced chemical equation comprises the same number of atoms for each element that occurs in the chemical reaction. We could obtain the number of atoms for a given element by multiplying the coefficient of any formula having that element by the subscript of the element in the formula. If we observe an element is noticed in more than one formula on a given side of the equation, we have to obtain the number of atoms and then added together.

Complete answer:
The given equation is,
$KO{H_{\left( {aq} \right)}} + {H_3}P{O_{4\left( {aq} \right)}}\xrightarrow{{}}{K_3}P{O_{4\left( {aq} \right)}} + {H_2}{O_{\left( l \right)}}$
Potassium hydroxide reacts with phosphoric acid to form potassium phosphate and water.
This is a double displacement reaction. We can write the unbalanced equation is,
$KO{H_{\left( {aq} \right)}} + {H_3}P{O_{4\left( {aq} \right)}}\xrightarrow{{}}{K_3}P{O_{4\left( {aq} \right)}} + {H_2}{O_{\left( l \right)}}$
Place a coefficient 3 before potassium hydroxide in the reactant side and place a coefficient 3 before water in the water side to balance the equation.
So, the equation becomes,
$KO{H_{\left( {aq} \right)}} + {H_3}P{O_{4\left( {aq} \right)}}\xrightarrow{{}}{K_3}P{O_{4\left( {aq} \right)}} + {H_2}{O_{\left( l \right)}}$
From the above equation, we can see that all other atoms are balanced.
So, we can write the balanced equation for the reaction of potassium hydroxide and phosphoric acid as,
$KO{H_{\left( {aq} \right)}} + {H_3}P{O_{4\left( {aq} \right)}}\xrightarrow{{}}{K_3}P{O_{4\left( {aq} \right)}} + {H_2}{O_{\left( l \right)}}$
In the above equation, the coefficient of water is three.

Note:
Accordance to law of conservation of mass, matter will not be created or destroyed. So, the weight of the products in a chemical reaction is the same as the weight of the reactants. The aim of balancing chemical equations is to have the equal quantity of each kind of element on each side of a chemical equation. When we are balancing a chemical equation, we can also find out the stoichiometric relationship between elements/compounds.