Question

Why did they choose x for unknown?

Answer 1

By newton's 2nd law,
summation force = ma
T-W = ma
865 -7.85x10^2 = (7.85x10^2 / 9.81) a(max)
80 = 80.02a(max)
max acc. = 1.00ms^-2
by s = ut + 1/2 (at^2)
45.7 = 0(t) + 1/2 (1)(t^2)
45.7 = 1/2 (t^2)
91.4 = t^2
t=9.56s