Note that you cannot use source transformations to complete the following problem. You just need to find the open circuit voltage and the short circuit current (then refer to the lecture slides to determine the Thevenin resistance). To find those quantities, you can use KCL and KVL to set up the equations and then solve the matrices for the two conditions.
Find the Thevenin voltage and resistance seen between terminals a and b:
80
-8A
30
a
5
100
264V
70
b