The sum of x – 3y + 2z and -4x + 9y – 11z is calculated as below: 

= (x – 3y + 2z) + (-4x + 9y – 11z)

= x – 4x – 3y + 9y + 2z – 11z

= -3x + 6y -9z

Now, The expression 3x- 4y -7z has to be subtracted from the resultant expression i.e. -3x + 6y -9z

= (-3x + 6y -9z) – (3x – 4y – 7z)

= -3x – 3x + 6y + 4y – 9z + 7z

= -6x + 10y – 2z