Your graph starts at (0, 6), because after 0 years (i.e. at the beginning) the tree is 6 meters high.
The next point would be (1, 6.5), because after one years the tree has grown half a meter, thus becoming 6.5 meters high.
Next, you have (2, 7): one more year passed, 0.5 meters gained in height.
You'll notice that the points align along a straight line. Prolong the line and you'll have the graph of the function
[tex]y=6+\dfrac{1}{2}x[/tex]
