She must start out going 8 miles per hour until the time elapsed is 1 hour at which time she drops 1.5 miles per hour. The same happens again in the second hour and the third.
The graph must have hours on the x axis and speed on the y axis.
The starting point is (0,8)
The graph must be
y = 8 - 1.5x
You need three points.
x y
0 8
1 6.5
2 5