The sets that a given number belong to is the number of “types” that the number is considered part of. So, the negative square root of 17 is a real number, so that’s one set; however, it is not an integer, a rational number, a whole number, or a natural number. Depending on how advanced you are—and how many number types you’ve been exposed to—the negative square root of 17 could also be considered a complex number (which includes the real numbers, as well as imaginary numbers like the square root of negative 1). There are two other sets the negative square root of 17 is part of: the quaterions and octonions, but most people aren’t even aware those classifications exist. So depending on your level of math, the answer is 1, 2, or 4.
