Problem # $1(20$ points$)$

A plastic bar ACB with two different solid circular cross sections is held between rigid supports, as shown in the figure. The diameters in the left section AC and right section CB are $50$ mm and $75$ mm $,$ respectively. The corresponding lengths are $225$ mm and $300$ mm $.$ Also, the modulus of elasticity is $6$ GPa $,$ and the coefficient of thermal expansion is $($:${?}^6$