Answer:
12 - 3z or 3 .9 - 3z
Explanation:
1 - if it is just a "9" and not a "0 .9" the answer is "12-3z". You will have to isolate the expressions with the "z" (the 2z and the z). They have a "-" before them, so they are negative expressions (-2z and -z). When the letter is alone, we supose that is a whole, not a decimal, expression, so it is like "1z". After all, you will have two negative expressions, the "-1z" and the "-2z", then you will just add "-1z" and "-2x" (-1z + (-2z) = -1z -2z = -3z
Now, you will need to add "9" and "+3" (9 + (+3) = 9 + 3 = 12).
Now, you have a "-3z" and the "12", and you will add they (-3z + 12 = 12 + (-3z) = 12 - 3z).
2 - But, if it is a "0 .9" ( "0 .9" is the same of " .9" and not a "9" the answer is "3 .9 - 3z". Now it is the same as the number 1 (just do the same from the "You will have" to the "-1z -2z = -3z".
Now, you need to add "0 .9" and "+3" ( .9 + (+3) = .9 + 3 = 3 .9).
So, you have the "-3z" and the "3 .9", and you need to add them ( 3 .9 + (-3z) = 3 .9 - 3z ).
I recommend you to study more of rule of signs, like (-) + (-) = (+) and things like this.
Hope that I could help you