If we arrange n resistors of value R ohms in series, the effective resistance:
n R ohms
if we arrange n resistors of value R ohms in parallel, then the effective resistance: R / n ohms
for getting effective resistance of 4 ohms from 6 ohm resistors, we can put two resistors in series, and their equivalent resistance = 12 ohms. Then connect the third resistance of 6 ohms in parallel.
equivalent resistance = 12 * 6 /(12+6) = 4 ohms.
if we connect two resistors in parallel then effective resistance = 6/2 = 3 ohms.
We connect the third resistance in series. The effective resistance: 3 + 6 = 9 ohms