What and what equal -13?

Answer 1

There are an infinitely many possible answers.

Using only whole numbers you have

. . . , -2 and -11, -1 and -12, 0 and -13, 1 and -14, 2 and -15, . . .

You can then consider numbers to 1 decimal place

and then 2 decimal places

and 3, and so on

etc.