http://www.mathsisfun.com/algebra/sigma-notation.html This link explains clearly what is that symbol. FYI

Mysql equivalent:

    Select count(*)
    from student
    where number<3000;

Answer is: 2

Syntax: σp(r)

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

σ(Number<3000)(Student) This Unary Relational Algebraic operation means, select number of students below 3000. So, the answer is 2.