Explanation:
it is the combination of operation and sign of the number.
if I subtract a positive number, then I simply subtract it. please consider that this is the same as adding the negative number of the same absolute value.
for example
3 - 2 = 1
this is the same as
3 + (-2)
this is also 1, as adding -2 is the same as subtracting 2.
maybe it helps, if you always put the sign in front of the numbers, even if it is positive :
+3 - +2 = +3 + -2 = +1
now if we subtract a negative number, the 2 "-" signs neutralize each other, and we get only "+".
+3 - (-2) = +3 - -2 = +3 + +2 = +5
this is like double negating a statement :
"I did not not eat that chocolate."
it means I ate the chocolate.
and the same happens when dealing with numbers and their signs.
so, we make this table :
++ = +
+- = -
-+ = -
-- = +