Theory:

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Rule 1: A variable is assigned to the value not known or to be found.
 
Let the number of apples that the boy initially had \(= x\).

Rule 2: Then, the expressions can be formed in by the conditions specified in the question.
 
Number of apples given to sister \(= 3\).
 
Number of apples left for the boy \(= 2\).

 
Number of apples he had initially \(–\) Number of apples given to the girl \(=\) Number of apples the boy is left with.
  
Rule 3: The expressions are finally equated with the values given in the question.
Number of apples the boy left with:
 
\(x - 3 =2\).
 
Thus, the equation set up is \(x-3 = 2\).