How tall would a tower need to be if the period of a pendulum were 30. seconds

We use the following expression

T = 2*pi * sqrt (l/g)

Where T is the period of the pendulum

l is the length of the pendulum

and g the acceleration of gravity

We solve for l

l = [T/2*pi]² * g = [30s/2*pi]² * 9.8 [m/s²] = 223.413 m

The tower would need to be at least 223.413 m high