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

$$\begin{aligned}\frac{1}{R_p} = \frac{1}{3} + \frac{1}{6} \\[0.5em] \frac{1}{R_p} = \frac{2 + 1}{6} \\[0.5em] \frac{1}{R_p} = \frac{3}{6} \\[0.5em] \frac{1}{R_p} = \frac{1}{2} \\[0.5em] \Rightarrow R_p = 2 \space Ω\end{aligned}$$

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