When the load is raised to 20 m it acquires a velocity v. When the cable becomes slack, the load continues its upward motion with a retardation g. Its instantaneous final velocity is zero. During that time it moves 4 m up. Using relevant kinematic equation find the velocity v.
This velocity v is the final velocity for the first part of the problem.
In that initial velocity is zero. Final velocity we found in the second part. Displacement is given. Find the acceleration. From that you can find the tension in the cable.