* Rubidium (Rb) is an alkali metal and forms a +1 cation (Rb+).

* Boron (B) is a metalloid and commonly forms a +3 cation (B3+) in compounds.

* Oxygen (O) is a nonmetal and forms a -2 anion (O2-).

To form a neutral compound, the charges of the ions must balance. The correct formula for the compound containing rubidium and borate would be Rb3BO3.

Now, regarding solubility:

* Rb3BO3 is a soluble compound.

* Generally, alkali metal compounds are highly soluble in water.

* Borate compounds are also generally soluble, with some exceptions.

Therefore, Rb3BO3 is a valid compound and is soluble in water.