That's a pretty difficult circuit I doubt the MCAT will ask about.
So the strategy is to combine components as you go around the circuit.
as long as there are no wires going in multiple paths in between resistors, you can combine them. For instance, you can't combine resistors b and d because you have alternate paths, they aren't in series with eachother.
Start from the battery and work our way around:
Resistors a and b are in series. Replace this with resistor Z with total resistance a + b = 6 ohms.
Redraw the picture so you have now one resistor instead of a and b.
See where else you can combine resistors and keep re-drawing, etc.
WIth the switches thrown in, thats a little more difficult and honestly, I doubt the MCAT ever throws in anything but a simple circuit.