How can i write 3 to "-1" and 2 to 1 , 1 to 3 , 5 to 0 and 7 to "-1" as an equation function

If you're just building a function, there's nothing stopping from simplify defining it as a function f : {1, 2, 3, 5, 7} -> {-1, 0, 1, 3} such that

In case it's not familiar to you, the notation f : A -> B simply means f is a function that maps elements in the set A to a single element in the set B ; I emphasize "single", because otherwise f would not be a function.