In French, the choice between "tu" and "vous" depends on the level of formality and familiarity with the person.
- "Tu" is used for informal situations, among friends, family, or people of the same age group.
- "Vous" is used for formal situations, when addressing someone you don't know well, someone older, or in professional contexts.
Assuming Robert is someone you don’t know well or is in a formal setting, you would likely use "vous." If Robert is a close friend or someone you are familiar with, you might use "tu."
Without additional context about your relationship with Robert, the safer choice would generally be "vous."