Next, I notice you wrote the expression Point[{0,9/8}]. There are a couple of things to note here. First, Point is used for graphics. The cartesian point as data would be represented as just {0,9/8}. Also, just evaluating that expression doesn't do anything for you, and you can see that it's just replicated in the output. You might want to assign it to a variable at some point, but you don't need it here in this cell.

And that's another thing. It's generally more organized to separate separate conceptual steps into separate cells. So, maybe you put your function definitions into a cell and then put your derivative expression in a separate cell. It's not necessary or critical, but it helps keep things organized and avoids unexpected interpretations.

Finally, in case you haven't discovered it yet, you'll want to look at ParametricPlot when you get to the point of plotting this function.