a) - ( -420 ) + ( -112 ) -420 + 12 b) - 678 - ( - 123 ) + ( - 123 + 678 ) - 2009

`a) -( -420 ) + ( -112 ) - 420 + 12`

`= 420 + ( - 112 ) + ( -420 ) + 12`

`= [ 420 + ( -420 ) ] + [ ( -112 ) + 12 ]`

`= 0 + ( -100 )`

`= ( -100 )`

`b) -678 - ( -123 ) + ( -123 + 678 ) - 2009`

`= ( -678 ) + 123 + ( -123 ) + 678 + ( -2009 )`

`= [ ( -678 ) + 678 ] + [ 123 + ( -123 ) ] + ( -2009 )`

`= 0 + 0 + ( -2009 )`

`= ( -2009 )`