Velocity = Change in Distance / Change in Time.

You need to use the units that are being asked, so if you want miles per hour, it is change in miles divided by change in time based on hours.

Example. Car is at 10 mile point at time 0. 10 minutes later it is at the 15 mile point.

Change in distance 15 - 10 = 5 miles

Change in time 10 - 0 = 10 minutes

velocity = 5 miles / 10 min = .5 miles/minute

You can change the units to miles per hour by dividing by 60 = 0.083 miles/hour. If you are working in metric units, the time is usually seconds and the distance meters.

Technically, this is really speed, since you did not give a direction. Velocity always has a direction too, so you you might say .083 miles/hour West if you have this information. If the distance is getting smaller, the final minus start number will be negative to reflect this fact, so you can have a negative velocity which gives direction. There is more to it, but that should hopefully help you out to get started with the basics.