Calculate the total resistance across AB :

From the circuit, the 3 Ω and 6 Ω are in parallel, and the 5 Ω is in series with that combination.

Step 1 — the parallel pair: R₁ = (3 × 6) ÷ (3 + 6) = 18 ÷ 9 = 2 Ω

Step 2 — in series with the 5 Ω: R = 5 + 2 = 7 Ω

Total resistance across AB = 7 Ω

Do the parallel part first, then add the series resistor — always work from the inside of the network outwards.

Check the parallel step: 2 Ω is less than 3 Ω, the smaller branch ✓. Adding all three in series gives 14 Ω, which is what you get by not reading the diagram.

For two resistors in parallel where one is a whole multiple of the other, the product-over-sum is quick: 3 and 6 give 18/9 = 2.