How to compute two objects of the same class?

I'm trying to compute the combined potential between the first and and last SuperHero instance of the _superHeroes array, and display the result. I created a method in the SuperHero class that computed the combined potential between two instances of a class. When I try to perform this method I get the following error:

TestSuperHero.java:20: error: cannot find symbol
    double n = _superHeroes.combinedPotential(_superHeroes[0], _superHeroes[2]);
                           ^
symbol:   method combinedPotential(SuperHero,SuperHero)
location: variable _superHeroes of type SuperHero[]
1 error

I would deeply appreciate some help! Thank you!

   import java.util.*;
    
    public class TestSuperHero
    {
       public static void main(String[] args)
       {
          //Welcome Message
          System.out.println("***** Welcome to the SuperHero Program ******\n");
          
          //Define Array of Three SuperHeros
          SuperHero [] _superHeroes = new SuperHero[3];
          
          //Initialize SuperHero Elements
          _superHeroes[0] = new SuperHero ("Poodle", 1000);
          _superHeroes[1] = new SuperHero ("HangNail", 200);
          _superHeroes[2] = new SuperHero ("WetSlipper", 50);
         
         //Create a Double Variable that will compute combinedPotential on 
         //_superHeroes[0] and _superHeroes[2]
         double n = _superHeroes.combinedPotential(_superHeroes[0], _superHeroes[2]);
       }
        class SuperHero
        {
           //Private Data Members
           private String _name;
           private double _kineticForce;
           
           //Constructor
           public SuperHero(String name, double kineticForce)
           {
              _name = name;
              _kineticForce = kineticForce;
           }
           
           /* Methods */
           
           //getName Accessor
           public String getName()
           {
              return _name;
           }
           
           //getKineticForce Accessor
           public double getKineticForce()
           {
              return _kineticForce;
           }
           
           //setKineticForce Mutator
           public void setKineticForce(double kineticForce)
           {
              _kineticForce = kineticForce;
           }
           
           public double damagePotential()
           {
              final double PI = 3.14159;
              double damagePotential = ((7.0/3.0) * PI) * (Math.pow(getKineticForce() / 2 , 5));
              return damagePotential;  
           }
           
           public double combinedPotential(SuperHero superHeroOne, SuperHero superHeroTwo)
           {
              double combinedPotential;
              double numerator = Math.pow(3 * superHeroOne.getKineticForce(), 2) 
                                - Math.pow(3 * superHeroTwo.getKineticForce(), 2);
              double denominator = superHeroOne.getKineticForce() - superHeroTwo.getKineticForce();
              
              combinedPotential = numerator / denominator;
              
              return combinedPotential; 
           }
           
           public String toString()
           {
              String superHero = "Name of SuperHero: " + getName();
              String kineticForce = "Kinetic Force of " + getName() + ": " + getKineticForce();
              return superHero + "\n" + kineticForce;
           }
        }