Again, let's take a look into what these numbers are:
Real numbers: It can be any numbers,both rational and irrational,basically every number can be qualified as real numbers.
Natural numbers: The number has to be integer and positive. e.g. 0,1,2,3,4
Integers: A number without fraction or demical place, it can be both positive or negative.
Whole number: The number has to be natural and cannot be negative.
Therefore, as -6 is negative number, it cannot be natural or whole,and belongs to real numbers and integers.
