Hello there,
This appears to be a tricky question. The 'tricky' part comes in the way the voltage source is specified, in that it is NOT equal to 30u(t), but it is actually 30u(-t), and that minus t is the little tricky part
What this means is that we end up having to analyze three different circuit states (not just two):
1. With the voltage source of 30v that has been on for a 'long' time (implies a dc source of 30v)
2. With the voltage source when t is between 0 and 1ms (this is a little bit tricky)
3. With the voltage source when t is between 1ms and 4ms, especially at t=4ms.
Now #1 and #3 are quite apparent, in that we start off with 30v and sometime later the switch opens and we have a discharging RC circuit, which should be easy to calculate once we consider the in between step #2.
However, #2 is a little tricky because we have to go back to the definition of what u(t) really is, and if we look at that u(-t) for any positive value of t we end up with zero (0v).
So, #2 means that for the time period between t=0+ and t=1ms we have to consider the 'source' a short circuit. That means we have a second time constant to consider, which is obtained from the "Req" value. What else this means is that we end up with a different starting voltage for the last step's RC discharge, which is not equal to 25v any longer (but much lower) because of this second step.
Given the above information, i think you should be able to figure out the solution they are looking for. If not, just yell back here again, and if so, let us know the answer you got so we can compare notes...
Hint:
You should be able to get the same value they show as the solution without changing anything about the circuit.