Phosphoric acid is a weak acid, while rubidium hydroxide is a strong acid.
H₃PO₄ + RbOH --> Rb₃PO₄ + H₂O
We get Rb₃PO₄ because PO₄ has a charge of 3-, that is PO₄³⁻. Rb has a charge of 1+. You give the subscript of one the charge of the other as this is an ionic compound. So you end up with Rb₃PO₄, a neutral compound.
Now let's balance the equation: H₃PO₄ + 3RbOH --> Rb₃PO₄ + 3H₂O
