Subtraction of signed numbers might be regarded as the addition of numbers of the opposite signs. For subtract signed numbers, reverse the sign of the subtrahend (the second number) and add.
For instance, one could treat his incomes for a provided month as positive numbers and his bills as negative numbers. There is difference of the two is his increase in cash balance. Assume he buys a window for $40. This provides a bill of $40 and adds as negative $40 to his cash balance. Now assume he returns this window to the store and the manager tears up his bill, subtracting the - $40. This is equal of adding +$40 to his cash balance.
a - b = a + (-b)
(+3) - (+5) = (+3) + (-5) = -2
(-4) - (-1) = (-4) + (+1) = -3
(-5) - (+8) = (-5) + (-8) = -13
(+7) - (-2) = (+7) + (+2) = +9