Calculate maximum current when charging a capacitor using a MOSFET
I'm going to make some possibly unwarranted assumptions:


The problem states that the cap is fully charged, therefore not charging any more under the given initial conditions.  Therefore the FET is supplying 1mA to the current source and there's no current in or out of the capacitor.
The gate voltage (2V) -Vc gives a Vgs that is 0.5V above threshold.  We can estimate transconductance then as 1mA per 0.5V.
Assuming roughly linear transconductance above threshold we can say that at 2.5V we would have 2mA of (initial max) FET current.  1mA into the cap and 1mA drawn by the current source.
If Vin were suddenly lowered to 1.5V FROM THE INITIAL CONDITIONS the FET would be at threshold where the current would presumably be &lt;&lt;1mA, so there's 1mA flowing out of the cap.