Answer
2300 m.
Work Step by Step
The original length of the steel band is $L=2 \pi R_{earth}$.
After the band is heated, the new length is $L+\Delta L=2 \pi R=2 \pi (R_{Earth}+\Delta R)$.
The change in radius is the height of the heated band above the surface of the Earth.
$$\Delta L=\alpha L\Delta T=2 \pi \Delta R$$
$$\Delta R = \frac{\alpha L\Delta T }{2 \pi }=\alpha R_{Earth}\Delta T$$
$$\Delta R =(12\times10^{-6}/C^{\circ}) (6.38\times10^6 m) (30C^{\circ})\approx 2300 m$$