Finding the Thevenin resistance
You might find it easier to forget you ever heard the word "Thevenin." You're trying to find a single voltage source and single resistor that would have the same open circuit output voltage and short circuit output current as the circuit in the problem (definition of Thevenin and Norton equivalents). There's no special rule that suspends Ohm's law - you surely do know how to solve the problem.  The 7.5k and 15k resistors are in parallel, so of course you can combine them, getting 5k. That leaves you with a voltage divider whose output is the Thevenin voltage. Then, to get the Thevenin resistance, first calculate how much current flows when the output of the problem circuit is short-circuited, and determine what size of resistor would give you the same short-circuit current from the Thevenin source.