How do you subtract an integer from another integer without using a number line or counters? give an example

When integers have the same sign always add the integers, either both have negative sign or positive sign.
for example +5 + 3 = +8
and -5 - 3 = -8 
but the answer will have the same sign as the numbers have like in the above example when both are positive answer is with positive sign and when both are with minus sign answer is with minus sign.
when  integers have opposite signs, always find the difference between them or do subtraction but the answer is with sign of the greater value integer, for example
-5 + 3 = -2, the answer is with - sign because greater number is 5 and it is with - sign.
+5 - 3 = +2
now the answer is with + sign because 5 is with + sign.

When i did the math i got 11 cm