Key concept Magnetic susceptibility measures the ability of a substance to take up magnetisation when placed in a magnetic field.
Given, $T_{1}=-73^{\circ} C =273-73=200\, K $
$T_{2} =-173^{\circ} C =-173+273=100 \,K $
$\chi_{1} =0.0075$
We know that, susceptibility of a magnetic substance,
$\chi \propto \frac{1}{T}$
So, $\frac{x_{2}}{x_{1}}=\frac{T_{1}}{T_{2}} $
$\frac{x_{2}}{x_{1}} =\frac{200}{100} $
$\frac{x_{2}}{0.0075} =\frac{2}{1} $
$x_{2} =2 \times 0.0075=0.015$