ε = {2, 4, 6, 8, 10, 12, 14, 16, 18}
A = {4, 8, 12, 16}
B = {10}
C = {6, 8}

Note that the contents of C are the values of x such that 15 < 3x < 30. So, we are looking at things like 3x = 18, not x = 18.

A n B = { } or the null set o/
DO NOT WRITE {o/} as this is not considered a null set.

A' = {2, 6, 10, 14, 18}
A' u C = {2, 6, 8, 10, 14, 18}
There are 6 elements inside, so...
n (A' u C) = 6

x is not an element of A' and x is an element of C

(If x is not an element of A', it must be an element of A, since there is no other way)

So, x must be an element of both A and C.

x = 8

So C={7,9} is not included in A' U C since there is no 7 and 9 in set E?

Yup, every of the choices HAS to be part of the universal set.

Ah i see