Since the axial structure is connected to rigid supports at $\mathrm{A}$ and $\mathrm{C}$, we can infer that the overall length of the axial structure will neither increase nor decrease when the $110 \mathrm{kN}$ load is applied at flange $\mathrm{B}$.

In other words, any deformation that occurs in member $\left(1\right)$ plus any deformaton in member $\left(2\right)$ must add up to be zero since the overall length of the axial structure cannot change.