1. The problem statement, all variables and given/known data

c=2.998 × 10^{8} m/s
t= 20.5 min (1230s)
x= ?

2. Relevant equations
my guess would be the standard velocity equation, v= x/t (x=vt)
but it appears i am wrong, is there another formula that includes velocity and time?

3. The attempt at a solution
v=x/t
x=vt
x= (2.998 × 10^{8} m/s)*(20.5*60)
x= (2.998 × 10^{8} m/s)*1230
x= 3.68751 x 10^{11} meters
x= 368754000 km

Hint: if a lighthouse flashes once a second, what is the difference between the time between the flashes as observed by someone moving towards it at speed v, and by someone else moving away at speed v?