what is the mistake of my calculation ,Transformation of the \$\Delta\$ to \$Y\$ circuit
It should be clear that the leftmost R is connected directly between 24+ and 24-. 

Try starting from here:







simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Schematic redrawn.

Hint: what is the voltage across r3?

Your answer of 18 &Omega; is correct.