11. 5(x - 4) - 4(x - 5)
=> 5x - 20 - 4x + 20
=> x
12. 2x - a - 5 + 2(-x -3)
=> 2x - a - 5 -2x - 6
=> -a - 11
13. -3(a - 3) - 2(a - 4)
=> - 3a + 9 - 2a + 8
=> - 5a + 17
14. 5(x+2) - 4(x-3)
=> 5x + 10 - 4x + 12
=> x + 22
16. 4x - 3(x - 2) - 6
=> 4x - 3x + 6 - 6
=> x
17. 4 - (8 - 9) . 3x
=> 4 - 24x + 27x
=> 4 + 3x
18. 3(3x -2y) - 4(2x - y)
=> 9x - 6y - 8x + 4y
=> x - 2y
19. 4(5a - 4b) + 2(a - 7b)
=> 20a - 16b + 2a - 14b
=> 22a - 30b
