Find the spring constant of the bimetallic bar shown in <image 1> in axial motion.
['$k_{eq}=4.80\\cdot10^{7}\\frac{N}{m}$', '$k_{eq}=5.80\\cdot10^{7}\\frac{N}{m}$', '$k_{eq}=6.80\\cdot10^{7}\\frac{N}{m}$']