If jack was traveling at 100 miles in 2 hours what was his velocity in miles per second

We don't know anything about the direction Jack was traveling, so we can't determine his velocity.  The best we can do is work with his speed.

Speed = (distance covered) / (time to cover the distance)

Speed = (100 miles) / (2 hours)

Speed = 50 miles/hour

The question has specified a different unit, so we'll need to convert the unit of our answer.  

Let's multiply the answer by (1 hour / 3600 seconds).  The numerator and denominator of this fraction are both the same quantity, so the value of the fraction is ' 1 ', and multiplying the answer by it won't change the answer in any way except the unit.

Speed = (50 mi / hr) x (1 hr / 3600 sec)

Speed = (50mi · 1hr)  /  (hr · 3600sec)

Speed = 50 mi / 3600 sec

Speed = 0.01389 mi/sec