What size resistor to reduce 4.5v to 3 v

From: BIGSteve (fitzgerald_s_at_hotmail.com)
Date: 01/27/05 

Date: 26 Jan 2005 16:41:17 -0800

got an old portable cd player needs 3vdc got a 4.5vdc car adapter need
to make the 2 work together. So how do i reduce the 4.5v to 3v can i
use a resistor? what size? thanks
also what size to go from 12vdc to 6vdc