The amount of time required to travel a certain distance is equal to quotient of distance and speed. We let x be the speed of Terry and Mark's will be x+2. The equation that would best represent the scenario is, 15/x = 21/(x+2) The value of x is 5. Thus, Mark's speed is 7 mph.
If you walk at a constant speed of x (I will let you determine x) x=step length you will the determine your time per yard... then multiply it by however you get it to 60.