On the top right side, 4 ohm and 8 ohm are in series (12 ohms), bottom two 12 ohm resistors are in parallel (12//12=6 ohms). This 6 ohm is in series with 24 ohms (30 ohms). 30 ohms is in parallel with 12 ohms (30//12=8.6 ohm). 8.6 ohm is in series with 2 ohm (10.6 ohm). 10.6 ohm is in parallel with 4 ohm and then series with 7 ohm [(10.6//4)+7]. Rab=9.9 ohm