How do you find the central idea of an article, passage, and/or paragraph?

What is the size of the matrix resulting from

[10 points, algebra 2]according to my answer key the restrictions are -1 and 4. but why not positive 1?

Aword processing program requires a 7- digit registration code made up of the digits 1,2,4,5,6,7 and 9 each number has to be used and no number can be used mor than once how many codes are possible