Show how would you join three resistors, each of resistance 9Ω so that the equivalent resistance of the combination is 13.5Ω?

Resistance of each resistor R = 9Ω
Two resistors connected across A and B are connected in parallel.
Equivalent resistance across A and B
RAB = R × R / (R + R) = R/2 = 9/2 = 4.5Ω
This equivalent resistance is connected in series to another resistor R.
So equivalent resistance across A and C
RAC = RAB + R = 4.5 + 9 = 13.5Ω