Calculate the total resistance across AB; between A and B are two parallel branches, one branch with a 3 Ohm and a 5 Ohm resistor, the other branch with a 4 Ohm resistor.

1/Rt=1/R1+1/R2
R1=3+5=8
R2=4
Rt=1/(1/8+1/4)=8/3 Ohms = 2.67 Ohms (2 D.P.)