Charles has 12 plastic cups of the plastic cups 3 were green 4 are red and 5 are blue she stack the cups into a single stack in random order whst i'd the probability that the cup on the top of the stack is not green

First, write out what you know.
Total: 12
Green: 3
Red: 4
Blue: 5

     Cool. Probability is "how likely", so how likely is it for you to pick a green cup? Well, if you pick any other cup, then it's not green, so you know they can't be the case. How many green cups are there? 3. 3 is the denominator of your probability.
     If you're picking the cups out of the total, which is 12, then your answer should be 3/12, which can be reduced to 1/4 :)