The platform on the phet simulation could only go 15 meters above ground.

calculate how far the cannon ball would go if we shot it off a platform of 100 meters tall?

120 m

Explanation:

Simply use the given equation that represents how far the cannon ball would go in terms of the height of the platform. That is: use 100 m for "x" (platform height) in the equation to find "y" (the distance the cannonball would go):

Notice also that both quantities in the graph display meters

Therefore: the cannon ball would go 120 meters

  distance on the y-axis and time on the x-axis.

explanation:

the instantaneous velocity = change in position/time taken

the slope of the graph should give instantaneous velocity.

  slope of a graph = change in value of y -axis / change in values of x -axis

  comparing both the equations

    change in value of y -axis = change in position

  change in values of x -axis = time taken

so position values should be on the y axis and time values should be on the x axis.

distance on the y-axis and time on the x-axis.