a) `-9sqrt(a) + 10sqrt(a) - 4sqrt(a) - 7sqrt(a)`

`= (-9+10-4-7) . sqrt(a)`

`= -10sqrt(a)`

b) `5sqrt(b) - 12sqrt(b) - 6sqrt(b) - (-3 sqrt(b))`

`= 5sqrt(b) - 12sqrt(b) - 6sqrt(b) + 3sqrt(b)`

`= (5 - 12 - 6 + 3) sqrt(b)`

`= -10sqrt(b)`

c) `-2sqrt(m) + 5sqrt(m) - 9sqrt(m) + 6sqrt(m)`

`= (-2 + 5 - 9 + 6) sqrt(m)`

`= 0 . sqrt(m)`

`= 0`

d) `-3sqrt(n) + 4sqrt(n) - 8sqrt(n) + (-5sqrt(n))`

`= (-3 + 4 - 8 - 5)sqrt(n)`

`= -12sqrt(n)`