Look at the numbers line above. Take any number
( 5 for example). If you want to add to it you move
In the right direction along the numbers line. For
example 5 + 2  = 7 and 5 + 4 = 9
If you want to subtract from it you move to the left
on the numbers line 5 - 2 = 3, 5 - 4 = 1 and 5 - 7 =  -2

If you take -5 the same rule applies. That is move
to the right when adding and to the left when
subtracting -5 +2 = -3, -5 -2 = -7 and -5 + 7 = 2.

Mixed signs
   + -   = minus                                   - - = plus
  5 - (+3 ) = 2                                    5 - (-3) = 8

    - + = minus                                    + + = plus  
   5 - (+5) = 0                                      5 + (+5) = 10
In words if the temperature  is +3 degrees and
drops by 7 degrees it would then be -4 degrees.
If the temperature was -2 degrees and rose by
6 degrees, it would then be + 4 degrees.

Likewise with multiplication and division
 + x + = plus                                   + x  -  = minus
 - x -   = plus                                    - x +   = minus
 + ÷ +  = plus                                    + ÷  -  = minus
 - ÷  - = plus                                      - ÷ +  = minus

(+5) x (+5) = +25                         (-5) x (+5) = -25

 (-5)  x (-5) =  +25                        (+5) x (-5) = -25

(+6) ÷  (+2)  = 3                                       (+6)  ÷ (-2) = -3