problem related to speed and velocity.

Velocity has two components, speed and direction.  Speed can bemeasured in whatever units you like, say miles per hour.  Direction is definedby the person doing the measurement.  So, if I define one direction to be positive I will have a positive velocity when travelling in thatdirection.  I will have a negative velocity when travelling in the opposite direction

you cannot have a negative speed many physical quantities, BY DEFINITION, cannot be negative some examples: speed, mass, measurements of size, the direction of time