A flexible rope of length 1.0 m slides from a frictionless table top. The rope is initially released from rest with 30 cm hanging over the edge of the table. Find the time at which the left end of the rope reaches the edge of the table.
The Attempt at a Solution
I have attached the solutions that the teacher gave us, but I don't understand the first step (that is all I attached). I understand all of the work after it, just not how they got the initial equation.