A bag contains 9 marbles 2 green 3 blue and 4 red what is the pobability of picking a green then a non-green?

Short answer: 14/81

There is a 2/9 chance of picking a green marble the first time.

Since this leaves 8 marbles (only one of which is green; you took the other one), that leaves an 7/8 chance of picking any other colour the second time.

To find the overall probability, you simply multiply the two values (2/9 * 7/8 = 14/81). This is simple to understand. You can only get a green marble every 2 out of 9 tries. For each of these 2 times, you have a 7/8 chance of getting a different colour the second time. So, you have a 7/8 chance every 2/9 times. Simply multiply them to get the total number of possibilities.