What subtraction problem equals -10?

Answer 1

There are infinitely many possible answers.

Take any number N.

Find M = N + 10

Then N - M will always be -10

For example, N = 3.75

then M = 3.75 + 10 = 13.75

and N - M = 3.75 - 13.75 = -10

Answer 2

One possible subtraction problem that equals -10 is 5 - (-15).