If a ball goes at 20 miles per hour what's the velocity

The velocity is

  (20 miles per hour) in the direction (that the ball is traveling this instant) .

If the speed OR direction of the ball changes, then the velocity has changed.

Alinear relationship would look like a straight line on the graph. it could have a positive negative or 0 slope. so it could be a line potentially going in any direction but it must be perfectly straight.