Answer
$T=2400^{\circ}C$
Work Step by Step
We know that:
$R=R_{\circ}[1+\alpha(T-T_{\circ})]$
This can be rearranged as
$T=T_{\circ}+\frac{1}{\alpha}(\frac{R}{R_{\circ}}-1)$
We plug in the known values to obtain:
$T=20^{\circ}+\frac{1}{0.0045}\times (\frac{140}{12}-1)=2400^{\circ}C$
