expansion

On heating the rod should have expanded to
l=l_0(1+\alpha \Delta \theta)

But there is a compression force acting on the rod which is preventing it from expanding.(Maybe the rod is fixed between 2 parallel walls or something like that).

\Delta l =\text{(Length of the rod due to the strain)-(Length of the rod if there was no strain)}
=l_0-l_0(1+\alpha \Delta \theta)=- l_0\alpha \Delta \theta

\text{Strain = }\frac{\Delta l}{l_0} = -6\times 10^{-4}