Use number line and add the following integers : (a) `9+(-6)` (b) `5+(-11)` (c) `(- c 1 )+ (-7)` (d) `(-5) + 10 ` (e) `( -1 ) + ( -2 ) + (- 3 )` (f) `( - 2 ) + 8 + (- 4) `