It is requiring you to use Suvat's equation.
x = x0 + v0*t + 0.5 * g * t^2.
Where
x [final position] = 0m, since we want the stone to fall to the ground.
x0 [initial position] = 45 m
v0 [initial velocity] = 0 m/s
g [acceleration of gravity] = 9.8 m/s
t = time that we want to find.
We are left with:
x = x0 + 0.5 * g * t^2. And therefore:
t^2 = (x - x0)/(0.5*g)
The square root of that number gives t.
