Use in or not in to fill in the gap: If 𝑍 is equal to the set of elements three, nine, and one, then three is blank 𝑍.
So we need to decide, do we need to use this symbol, where three is in 𝑍, or use this symbol, three is not in 𝑍. We know that 𝑍 contains the elements three, nine, and one. So since three is indeed an element of 𝑍, we need to use this symbol. Because it means in where the other one means not in. And three is included in the set 𝑍. So this would be the symbol that we should use.