Just like when you are told

You put down the second one, 10, than a minus sign, 
and then the first one, 4, like this:

10-4

and then simplify to get 6.  

So when you are told
Subtract (-3x-3y+4z) from (9x-4y-5z)
(after putting parentheses around them)

You put down the second one, (9x-4y-5z), than a minus sign, 

and then the first one, (-3x-3y+4z), like this:

(9x-4y-5z)-(-3x-3y+4z)

Now you simplify by first removing the parentheses,

remembering that when a parentheses is preceded by

a + you just erase the parentheses, but when a

parentheses is preceded by a - you nust change the

sign of every term inside when you erase the

parentheses:

9x-4y-5z+3x+3y-4z

Then you combine like terms:

           9x and +3x to get 12x

          -4y and +3y to get -1y or just -y

          -5z and -4z to get -9z

So the final answer is

12x-y-9z

Edwin