Consider a free-body diagram of beam AB. There are three unknown forces acting on the beam: Ay, By, and Cy.
With three unknowns and only two equilibrium equations
(i.e., Σ Fy = 0 and Σ M = 0), we cannot solve for the unknown forces using statics alone. To solve for the three unknown forces, we must develop a third equation that includes the unknown forces.
Since statics does not provide enough information to solve for the reactions, this type of problem is termed statically indeterminate.