#### Answer

(a) The tower should be 78 meters high.
(b) The impact velocity is 39 m/s

#### Work Step by Step

(a) We can find the vertical height $y$ that an object in freefall drops in 4.0 seconds.
$y = \frac{1}{2}a_y~t^2$
$y = \frac{1}{2}(9.80~m/s^2)(4.0~s)^2$
$y = 78~m$
The tower should be 78 meters high.
(b) We can find the impact velocity $v_y$.
$v_y = v_{y0}+a_y~t$
$v_y = 0 + a_y~t$
$v_y = (9.80~m/s^2)(4.0~s)$
$v = 39~m/s$
The impact velocity is 39 m/s