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This link explains clearly what is that symbol. FYI
Answer is: 2
Where, σ represents the Select Predicate, r is the name of relation(table name in which you want to look for data), and p is the prepositional logic, where we specify the conditions that must be satisfied by the data. In prepositional logic, one can use unary and binary operators like =, <, > etc, to specify the conditions.
Font size is too little in the expression
This Unary Relational Algebraic operation means, select number of students below 3000. So, the answer is 2.