Partial credit!
First, in order to satisfy the no ripple assumption, Cin has to be HUGE, like >10,000uF!
The minimum allowed voltage at the regulator input is -5+(-Vdo), where Vdo is the DropOut Voltage of the regulator, typically ~2V.
Cin has to supply all of the output current for ~10msec (at 50Hz line freq), so the size of Cin depends on load current, and what the transformer secondary voltage is.
You have two variables which can be traded off; the size of Cin and the secondary voltage. Usually, you pick an off-the-shelf transformer which dictates Vsec, and then you compute the size of Cin.
Tell me what transformers you have available, and I will show you how to calculate Cin.