$$\begin{gathered} 20-\mathrm{mm} \mathrm{diameter} \mathrm{steel} \mathrm{rod} \\ \mathrm{A}=314.16 {\mathrm{mm}}^2 \\ \mathrm{E}=200 \mathrm{GPa} \\ \mathrm{\alpha }=12.0\times {10}^{-6}{/}^{\circ }\mathrm{C} \end{gathered}$$

Consider the overall geometry of the assembly and how that geometry dictates the deformation that can occur.

Since the assembly is connected to rigid walls (meaning that the walls do not move), the overall deformation must equal zero. In other words, $\mathrm{\delta }=0$.