Can a Dependent source be dependent on itself?
There are several ways of determining the Thevenin equivalent resistance. One way is to open the output and measure measure the open circuit voltage and resistance. However, that doesn't work here because you end up with a nonsense term 4 ib = ib. So you have to use a different method. I think they penalty want you to use the "test source" technique here, where you attach a test voltage source to the output.