In the movie Contact, Jodie Foster's character detects a radio signal from Vega. Vega is 2.5 times 10 to the 17 meters from our solar system. If we received a radio message from this star today, how long ago would it have been sent?

distance = rate * time

You know the distance, and you know the rate (the speed of light, or around 3x10^8 m/s).

Just plug in the numbers and solve for time. That will give you the answer in seconds (and it will be a really big number). Do you know how to then convert from seconds to something more reasonable (like years)?