Light year is the distance traveled by light in one year. 1 light year = 9.4607x10^12km. So 150 light years = 150x9.4607 x 10^12 = 1.419x10^15km = 1.419 x 10^18 m. Therefore, time = distance/speed = 1.419 x 10^18/3 x 10^8 = 4730000000s. Hope it helps.
