How to do i fix this code im not good at using function, im trying to pass a statement from switch to my double passengersR i always come out with a error

this output:
Name passenger:
Enter number passenger:
Choose from A to D
if choose A
Total Regular Passenger: 6.19624e-312

did i do something wrong im not really good at function and im trying to learn how to pass switch to void function

Code:
#include<conio.h>
#include <iostream>
#include <Iomanip>
#include <string>

using namespace std;

double passengersR ()
{
       
double m;
int num;

m=120*num;
return m;

}

int main()
{
    double passengersR ();
    double m;
    char x ;
    string Name;
    float tickPrice;
    int  num;
    string morePassengers,name;
    
    cout<<"Enter Passenger Name:";
    cin>> Name;
    cout<<"Enter Number of passenger ";
    cin >> num;
    cout<<" A.Regular B.Student  C.Senior D.Children ";
    cin>>x ;
         
    cout << endl << endl; 
    cout << x << endl ;
    
    
    switch (x)
    {
    case 'a':
    case 'A':       
    cout<<"Total Regular Passenger:" << m << endl;      
    break;
    }
    
    
    
    
    getch();

}