Thinking of subtracting negative numbers as the difference between temperatures can be helpful...
5-2 is the difference between 5 and 2 degrees, which is 3 degrees.
There is a 6 degree difference between between 5 and -1,
5 degrees down to zero and another degree down to -1.
That's a drop of 6 degrees.
So 5-(-1) is that difference which we know is 6,
subtracting the lower temperature from the bigger one.
Since 5+1=6, we can say 5--1=5+1,
so 1--1=1+1 and so on...
-2-3 is subtract 2, then subtract 3, which is subtract 5 or -(2+3)
-2--3=-2-(-3), the difference between -2 degrees and -3 degrees which is 1 degree
as -2 is greater than -3,
so -2--3=1=3-2 or -2+3 etc