Find the Resistance between Two Terminals
The equivalent resistance is just \$V/I\$ for a source applied across the given terminals.

So you can imagine a voltage or current source connected between terminals a and d, and use whatever method you like (KVL or KCL) to solve for the current or voltage.

Alternative solution: You can use a symmetry argument to show that if you apply a voltage between a and d, one of the resistors will have no current through it, and therefore can be removed without changing the circuit. Once this resistor is removed you can use parallel and series combinations to find the equivalent resistance of the remaining resistors.