Which set of numbers does -3.5 belong to?

To any set that contains it!

It belongs to {-3.5},

or {-3.5, sqrt(2), pi, -3/7},

or all numbers between -43 and 53,

or multiples of 0.5,

or rational numbers,

or real numbers,

or square roots of 12.25,

or complex numbers,

etc.