I want to know the meaning of the following verse:....
"And if the people of the towns had believed and had the Taqwa (piety), certainly, we should have opened for them blessings from the heaven and the earth, but they believed (the Messengers). so we took them (with punishment) for what they used to earn (polytheism and crimes)."
In some countries like Kenya and America their National anthem contains the following sentences "God bless America" (U.S) and "Oh God bring us your Blessings" (Kenya).
My question is does this verse of Qur'an means that these prayers by the people will be answered if they have Faith and Piety?

Praise be to Allah, the Lord of the Worlds; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

Allah informs us that if the people of villages had believed in Him, He would have provided them sustenance and opened the doors of His blessings from the sky and the earth.  On the contrary, if they disobey Him and disbelief in Him He will withhold His blessings from them due to their disobedience.  This is a general rule, but sometimes He even pours His blessings upon His disobedient servants as an Istidraj which means (Allah pours His blessings despite the disobedience of His slaves to a certain period and when they think that Allah favors them He catches them and punishes them).  Allah Says (interpretation of meaning): {So, when they forgot (the warning) with which they had been reminded, We opened to them the gates of every (pleasant) thing, until in the midst of their enjoyment in that which they were given, all of a sudden, We took them in punishment, and lo! They were plunged into destruction with deep regrets and sorrows.}[6:44].

However, Allah sometimes deprives pious people of His blessings to put them on trial and examine their faith as He states in His Qur'an {As for man, when his Lord tries him by giving him honour and gifts, then he says (puffed up): "My Lord has honoured me." But when He tries him, by straitening his means of life, he says: "My Lord has humiliated me!" ….}[89:15-17]. 

As regards the Du’a that is in American or Kenyan national anthems it is not an evidence of their Islam as long as they do not believe in the Prophet Muhammad (Sallallahu Alaihi wa Sallam) and in Qur'an, and do not claim that their religion is Islam.  Allah Says (interpretation of meaning): {And whoever seeks a religion other than Islâm, it will never be accepted of him, ….}[3:85].

Since they do not accept Islam they are Kafir and their Du’a will not benefit them.  Allah Says (interpretation of meaning): {….and the invocation of the disbelievers is nothing but an error (i.e. of no use).}[13:14].

Dear brother, do not forget what Allah mentioned about Kuffars: {Let not the free disposal (and affluence) of the disbelievers throughout the land deceive you. ……}[3:196].