Step-by-step explanation:
we assume that "thrown" simply means "released".
that means only gravity is pulling the ball down.
if it were actually "thrown down, there would be a different force and a different initial speed applied.
since you used meters for the height of the building, I also assume that we use only the metric system for everything. and that we are talking about the Earth and Earth's gravity.
so, we know :
the speed of the falling ball as function of time is
v = 9.8 m/s² × t
and the distance traveled is
d = ½ × 9.8 m/s² × t²
we know d = 45 m
and we get
45 = ½ × 9.8 m/s² × t²
90 = 9.8 m/s² × t²
t² = 90/9.8 = 9.183673469...
t = sqrt(9.183673469...) = 3.030457634... sec
it took the ball 3.030457634... seconds to reach the ground.