The resistors 3Ω and 6Ω are in parallel. If their equivalent resistance is Rp, then
‌‌

1

Rp

=‌

1

3

+‌

1

6

‌‌

1

Rp

=‌

2+1

6

=‌

3

6

‌‌

1

Rp

=‌

1

2

or Rp=2Ω
Now, Rp and 5Ω are in series,
So, R s‌=2+5
‌=7Ω
Thus the total resistance across AB=7Ω