Thevenin Equivalent with dependent sources
You can replace the 9*Ia dependent supply with a circuit that produces the same current, that is 9 repeated branches in parallel consisting of a 20 V supply in series with a 50 k resistor and then simplify.

But that is a long, improductive way to do it.

A better way to find the equivalent resistance is using the method of test supplies between A and B points. Remember to turn off the independent supplies. This is more useful as sometimes finding the circuit can be difficult.

If you want to find the short circuit current between A and B, it must be the current going through the short branch only, so keep the dependent supply and the 5 k resistor in your analysis.