Speed =

Speed =   (Substitute distance = 3 miles and time = 6 seconds)

1 hour = 60 minutes

1 minute = 60 Seconds

So, 1 hour = 60 × 60 Seconds

1 hour = 3600 seconds

Speed =

Speed =  ×      

Speed =   ×      (Cancel the common unit seconds)  

Speed =  

Simplify, Speed =

Speed = 1800 miles per hour

The speed is 1800 miles per hour.