Calculate the total resistance across AB:

This circuit consists of two parts. In the first part, the resistances 3 ohm and 6 ohm are connected in parallel. If R_p is the equivalent resistance then

$\dfrac{1}{R$p} = \dfrac{1}{3} + \dfrac{1}{6} \\[0.5em] \dfrac{1}{Rp} = \dfrac{2 + 1}{6} \\[0.5em] \dfrac{1}{Rp} = \dfrac{3}{6} \\[0.5em] \dfrac{1}{Rp} = \dfrac{1}{2} \\[0.5em] \Rightarrow R_p = 2 \space Ω

Therefore, R_p = 2 Ω

In the second part, R_p (i.e., 2 ohm) and 5 ohm are in series. If the equivalent resistance = R_s the we get,

R_s = 2 + 5 = 7 Ω

Hence, equivalent resistance across A and B = 7 Ω