For each cube there are 6 outcomes, so there are 6*6 = 36 outcomes for two dice. Visually you can have a table that is 6 rows and 6 columns leading to 36 cells overall.
There is a 1/4 chance of Missy rolling a prime number on both cubes.
Rolling one 6-sided cube, Missy has a 3/6 or 1/2 chance of landing on a prime number because the prime numbers between 1 and 6 (inclusive) are 2,3 and 5. When Missy rolls 2, 6-sided cubes, however, the chances of that happening lessens, and we multiply 1/2 by 1/2 as a result. Therefore, when Missy rolls two 6-sided cubes, there is a 1/4 chance of Missy rolling a prime number on both cubes.
The probability that Rachel will win the game p(E) = 1/18 =0.055