A steel wire $3.0\ \text{m}$ long, with a cross-section of $1.5\ \text{mm}^2$, holds a $12\ \text{kg}$ hanging sign. By how much does it stretch? ($Y = 2.0 \times 10^{11}\ \text{Pa}$, $g = 9.8\ \text{m/s}^2$.)