+800:−4:a?0 +800:-4:a\text{?}0 +800:−4:a?0
Note that in the first stage we are dividing a positive number by a negative number:
+:−=− +:-=- +:−=−
Now the exercise is:
−:a?0 -:a?0 −:a?0
Since we don't know whether a is a positive or negative number, we cannot determine the sign.
It is not possible to calculate