Determine the Thévenin equivalent circuit for the portion of the
network external to points a and b?

if we make the points a and b open circuit then we have open circuit voltage as (30/90)30=10v. However i am not sure if this is correct because we have another voltage source of 20V.

 

Determine the Thévenin equivalent circuit for the portion of the
network external to points a and b?

if we make the points a and b open circuit then we have open circuit voltage as (30/90)30=10v. However i am not sure if this is correct because we have another voltage source of 20V.

You're on the right track, but you do have to consider the other voltage source of 20v. Aren't these two sources (10v and 20v) in series? Be careful to observe the voltage polarities.