Use the definition of normal strain to determine the change in length of post $\left(2\right)$ corresponding to the measured strain of $–1,800 \mathrm{\mu \epsilon }$.

$$\begin{gathered} ∆\mathrm{L}=(-1,800 \mathrm{\mu \epsilon }) \frac{1 \mathrm{mm}/\mathrm{mm}}{1\times {10}^{6 }\mathrm{\mu \epsilon }}(600 \mathrm{mm}) \\ =-1.08 \mathrm{mm} \end{gathered}$$