12 out of 18. This is because there is 6 red marbles and 18 of them all together. The question states the probability of the red marble NOT being chosen therefore you need to take away 6 from 18 leaving you with 12. I hope this made sense :)
The cube has 6 this same faces.
Therefore the surface area is equal 6 · 7cm² = 42cm²
13 m is the volume of the prism