A resistor alone is not sufficient, unless your circuit has a constant and known current draw. Recall that V = IR, so you can calculate the voltage drop (V) across a resistor R only if you know the current I through the resistor.If you are needing some "standard" voltage output (like 3.4V) you're better off using a three-pin voltage regulator with a heat sink.
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Well, do you know the supply current? If so, use R = V/I (where V is the voltage you want, and I is the supply current) work it out and that should give you the desired resistor value.
You could just tell us what you want to run @ 3.4V and from which 5 or 7.5V supply?