The correct answer is letter C :)
The expression 6-13 can also be interpreted as:
Indicating that we start in positive 6 then moving
it by negative 13. In the number line, negative
means we move the value of the integer to the left,
and when positive, we move the value of the
integer to the right.
On the given illustration: "Starting from 6, moving
it 13 to the left, we get -7"
