Each diode is modeled as a piece-wise linear device with the following parameters:
Vd = 0.7
Rd = 26Ω

This means an ideal diode in series with a 0.7 voltage source in series with 26 ohm resistor, right?

Task:
a) Sketch Vs and Vo as a function of time.
b) Calculate the ripple voltage of O v and determine what percentage it is of the peak output voltage.

 

For the first replace the diodes with their equivalent circuit and use a simulation software to sketch it for you.

For the second you will need a bit of math.

To an approximation:

Vodc=Vpeak/[1+1/(4fCR)]

Vripple=Vdc/(2fCR)

f=mains frequency