We can use the equations of motion to solve this problem. The distance traveled can be given as:

`s = ut + 1/2 at^2`

where, s is the traveled distance, u is the initial velocity, t is the time taken and a is the acceleration of the object. 

In the given case, the penny is 'dropped'. This means that its initial velocity( u) is zero, that is u = 0. Since the penny has been dropped from a rooftop, only the Earth's gravity will cause any acceleration on it, which is nothing but acceleration due to gravity, g (= 9.81 m/s^2). And the time taken to hit the ground is 6 seconds. Hence the height of the building is:

`s = (0) xx 6 + 1/2 xx 9.81 xx 6^2 = 176.58 m`

Thus, the building is 176.58 meters tall.

A meter has 3.28 feet. Hence the height of the building is 579.18 feet.

Hope this helps.