Contrapositive -> "We have already sent Our messengers with clear evidences and sent down with them the Scripture and the balance that the people may maintain [their affairs] in justice. And We sent down iron, wherein is great military might and benefits for the people, and so that Allah may make evident those who support Him and His messengers unseen. Indeed, Allah is Powerful and Exalted in Might."
Discussion
The contrapositive of the statement "We have already sent Our messengers with clear evidences and sent down with them the Scripture and the balance that the people may maintain [their affairs] in justice. And We sent down iron, wherein is great military might and benefits for the people, and so that Allah may make evident those who support Him and His messengers unseen. Indeed, Allah is Powerful and Exalted in Might." would be "If Allah is not powerful and exalted in might, then we have not sent our messengers with clear evidences and sent down with them the Scripture and the balance that the people may maintain [their affairs] in justice. And We have not sent down iron, wherein is great military might and benefits for the people, and so that Allah may make evident those who support Him and His messengers unseen."