According to article one of the Constitution what power does the legislative branch have?

ability to establish "necessary and proper" clause

Explanation:

Government's duties are very complex. It's impossible to write all of government powers to fit in the length the of the constitution. This was the reason why the founding fathers added the power to establish "necessary and proper" clause.

This clause gave the legislative branch to establish all legislations that are necessary in order to carried out the main powers displayed within the constitution. This gave the legislative branch with a lot of flexibility to design their legislations depending on specific situations that United States face.