Ah Razavi's old intuitive exercises. It's commonly discouraged to give direct answers to these problems and let the student use critical thinking. But if you'd like the easy way there's always a solutions manual somewhere in the internet. But before you go there try solving it with a few hints.
For (a)
(1) capacitor charging and discharging is based on Cdv/dt. so we can surmise that Ix=-Cdvx/dt (negative for discharging)
(2) use node equations. Ix=I1+Id
(3) since there's a current source present, C1 charges indefinitely (so keep in mind transition from saturation to triode and take note on continuity)
(4) for razavi I think Vt=0.7V so keep in mind on drain current equation when transitioning from saturation to triode.
for (b)
(1) same charging/discharging principle.
(2) you'll see that Vgs=2-Vc1 so at some point the device will turn off and values will theoretically remain constant with time.
(3) Here c1 is charged by the drain current so Id=Cdvc1/dt.
Cheers. Happy Solving!
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Wait, C1 is 1V and C2 is 3V? then (b) will first reverse polarity. (i.e. drain and source will exchange)