Answer:
F = M a is the vector equation involved
F = T - M g are the forces acting on the elevator (scalar equation)
T - M g = M a
T = M (a + g) remember this a scalar
If a is slowing down then it must have a positive acceleration upwards
Therefor the tension in the cable must be greater than zero
When the tension increases to M g, a has increased to zero
For a to be zero, no acceleration, T = M g