what is the interval of 2 5 6 3 1 4 7 and how do you figure this

An interval represents a set of numbers. You can reorder the set of numbers in trying to establish a logic as to how to describe them.

Reordering it in numerical order, the set of numbers A={1,2,3,4,5,6,7}, i.e. all integers between 1 and 7.

While intervals of real numbers (ℝ) are represented in the form [a,b], intervals of integers (ℤ) are represented as follows: [a..b].

Thus the given set in interval format would be
[1..7]