Answer:
-2
Step-by-step explanation:
Here is a number line:
<-------+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+-----+------>
-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6
Start at 3, marked with an S below:
<-------+-----+-----+-----+-----+-----+-----+-----+-----+-----S----+-----+-----+------>
-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6
Since you are adding a negative number, -5, count 5 numbers to the left starting on P which corresponds to 3.
5<---4<---3<----2<---1<---Start
<-------+-----+-----+-----+-----E-----+-----+-----+-----+-----S----+-----+-----+------>
-6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6
You end up at E which corresponds to -2.
Therefore, -5 added to 3 is -2.
I don't know how your teacher can say that 3 + (-5) is not -2 since -2 is the correct answer. Perhaps you should check on the question to make sure it really is 3 + (-5) and not something else.
