(a)
Given data:
The given set is {-1, 6, 3,1, a}.
The set {a, 1, 7} is not subset of the given set.
![\mleft\lbrace a,\text{ 1, 7}\mright\rbrace⊄\mleft\{-1,6,3,1,a\mright\}](https://img.qammunity.org/2023/formulas/mathematics/college/ytowbkr709pp98uq2ro3nn50nh5g1ohkkp.png)
Thus, the symbol must be used is ⊄.
(b)
Given data:
The given set is {3, 5,7,9}.
The number 3 is belongs to the given set.
![3\in\mleft\lbrace3,\text{ 5, 7, 9}\mright\rbrace](https://img.qammunity.org/2023/formulas/mathematics/college/p1qqzssa3wx2nd74u9mqnehkdc8dknd4c3.png)
Thus, the symbol used in the above expression is ∈.