non static method ... cannot be referenced from a static context

Here is the error message

non static method hero(double,double,double) cannot be reference from a static context

Here is the class method.

class MyMath {
   double hero(double n1, double n2, double n3)
   {
     double n4;
     double n5;
     n4 = (n1 + n2 + n3) / 2;
     n5 = Math.sqrt((n4 * (n4 - n1) * (n4 - n2) * (n4 - n3)));
     return n5;
   }
 }
 

Here is the main program

double length_of_a;
 double length_of_b;
 double length_of_c;
 double area_of_triangle;
 
 area_of_triangle = (double) MyMath.hero(length_of_a,length_of_b,length_of_c);
 

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