To set up an equation for the acceleration constraint involving a constant length string, the relationship can be expressed as L = 2x_a + y_b + C. To differentiate this equation, one must consider the differential quotient of a constant, which is zero. The discussion emphasizes the need to relate velocities and accelerations to the first and second time derivatives of x_a and y_b. Understanding these relationships is crucial for deriving the acceleration constraint. The focus is on establishing a clear connection between position, velocity, and acceleration in the context of the problem.