Solve for 2x- 15=2x-6​

the average speed for the object is V=d/t

1) for t=0 sec  to t=8 sec
 I take two points of the graph
point A(0,0)
point B(8,200)
the average speed for the object= slope of the equation of the line
m= (y2-y1)/(x2-x1)=200/8=25  m/sec
V=25 m/sec

2) for t>8 sec to t=12 sec
the slope=0
therefore
V=0

3) for t>12  sec to t=20 sec
 I take two points of the graph
point A(15,400)
point B(18,500)
the average speed for the object= slope of the equation of the line
m= (y2-y1)/(x2-x1)=100/3=33.33  m/sec
V=33.33 m/sec