'Exert' means to apply force on influence over something else, this can be in a negative or positive way. In this quotation, it means that language, if used correctly, has the ability to express a 'hidden power'. It's also worth mentioning that the simile is relevant as the moon has an influence on the level of tides on Earth.
The correct answer should be Should this land be used to build a library?
The land belongs to the country, and the government has to decide what to do with that land, because it is its property.
You add a comma to prepositional phrase when you want to tell more about the adjectives and adverbs.