Question

how would you join the three resistors each of resistance 9 ohm so that the equivalent resistance of the combination is 13.5 ohm

Answer 1

first two resistors will be connected in parallel so that their resultant will be
1/R=1/9+1/9
R=4.5 ohm
And the third resistors will be connected in the series with the combination of first two resistors so the equivalent restance will be
R=9+4.5
R=13.5 ohm

......hope this will help u