Not really a call for help, more than a confirmation/debunk of my work

Morten Andersen said:

Alright. the original assignment is the picture below. The information given is that the voltage over Vref is equal to the voltage over R2.
The point of the assignment is to find the value of R3, so that you get the desired wattage over IRF530, and then by changing the size of R3 you should get different wattage use of the IRF530. i'm trying to determine how to set up the calculations, so it's understandable for myself.

Click to expand...

The original sketch you had used a fixed value for Vdd of 20 V while the problem statement appears to indicate that Vdd could vary over the range from 10 V to 30 V. That's a problem because the opamp circuit will adjust the gate voltage to get a constant current through the MOSFET but the power dissipated in it is the current multiplied by the voltage across it.

What you could do would be to make a plot of the value of R3 on the one axis and the value of Vdd on the other and then plot a line for 10 W dissipation (and you could add other lines for other dissipations, as well). Once you figured out how to find R3 for a particular value of Vdd in order to achieve a particular power dissipation, setting up a spreadsheet to produce such a plot would be a pretty simple matter.

So I would still recommend that you focus on the questions I asked in Post #7, assuming Vdd = 20 V, the power dissipation is 10 W.