When we have a graph with distance and time, the velocity will always be the slope of the graph, remember the definition of velocity
[tex]v=\frac{\Delta S}{\Delta t}[/tex]If S is on the y-axis and t on the x-axis it will be the same thing as the slope! Therefore we just need to find the slope of this line.
Looking at the graph we can see that at x = 40 we have y = 300, and x = 80 implies y = 600. Using the formula:
[tex]\begin{gathered} v=\frac{600-300}{80-40} \\ \\ v=\frac{300}{40} \\ \\ v=\frac{30}{4} \\ \\ v=7.5\text{ km/s} \end{gathered}[/tex]The velocity of the Hubble telescope is 7.5km/s
[tex]v=7.5\text{ km/s}[/tex]