First, learn how to draw a schematic. What you are presenting is unreadable. Paper and pencil might help, before putting something onto a printout.
Now, decrypting your "schematic", we have what is a MOSFET source follower.
Due to the feedback, the opamp will try to keep the voltage difference at the inputs at 0.
The noninverting input is at 3.5 V.
The inverting input should also be at 3.5 V, which means that 35 mA has to flow through the load resistor RL.
The opamp will drive the MOSFET gate voltage to a point, where this condition is fulfilled.
You do not need to think about gate cutoff voltage, unless it moves outside supply/opamp output voltage limits.
Simple, no?