Ques. about Concept of Thev Analysis

Thanks for the help,think you could check over the other problem briefly.I understand if you don't want to lol. In part b/c of my ignorance of certain concepts. But I believe I understand the concept of the resistors being in series in a wheatstone if theres no current in my second problem when trying to calculate Voc,and I now think I have a understanding of getting the voltage from Va -> Vb for Vab.