In given circuit, the resistance 3ohm and 6 ohm are in parallel.
So, the equivalent resistance between 3 ohm and 6 Ohm parallel formula will be
R1 equivalent = 3*6/(3+6) = 18/9 = 2 ohm
Now R1 equivalent and 5 ohm resistance are in series.
So Total equivalent resistance between A and B = 2+5 = 7 ohm