bool Complex::equal(const Complex &x )
{
   return ( realPart = = x.realPart &&
                  imaginaryPart = = x.imaginaryPart);
}

Here const is not really essential but its use is a good practice because you do not want to change the object's internal variables through these functions.

If you have an implementation with bool Complex::equal(const Complex x) that is fine. In that case try, to see the difference.

Complex Complex::add(const Complex &x )
{
   // your code
}

class Complex {

// some members are public and some are private
  

   Complex add(const Complex & );        
   bool equal(const Complex &);            
  
    // Add other members
};