The correct answer is "ganaron". Ayer, Roberto y Carlos ganaron 12.000 pesos. In English will be: Yesterday, Roberto and Carlos won 12.000 pesos.
To help you out a little bit more, please check the following:
tu ganaste (you won)
nosotros ganamos (we won)
ellos ganaron (they won)
el/ella gano' (he/she)
Good luck!
