Use the fact that the speed of light in a vacuum is about 3.00 × 108 m/s to determine how many kilometers a pulse from a laser beam travels in exactly twelvehours.

The first thing that needs to be done is to find everything in the same units.  12 hours becomes 43200 seconds.  Then find the distance traveled by light in that amount of time.  Using the formula v=d/s, manipulate it so it looks like d=v*s.  Then plug in the values: d=(3x10^8)*43200,  d=1.3x10^13m.  But you need to find this in kilometers.  To do this, simply divide your answer by one thousand.  Thus, a laser beam would travel 1.3x10^10 kilometers in 12 hours.