float r=a*b*c*d;
float x = r;
float y = 1;
while (y*y<=x)
{
y=y+1;
}
y=y-1;
cout<<y;
float result;
result=y;
return (result);
}
int main()
{
float a, b, c;
cout<<"enter first side of triangle: ";
cin>>a;
cout<<"enter second side of triangle: ";
cin>>b;
cout<<"enter third side of triangle: ";
cin>>c;
float s=(a+b+c)/2;
float d=s-a;
float e=s-b;
float f=s-c;
float area_triangle = trianglearea (s*d*e*f);
cout<<area_triangle;
while calling the function trianglearea, you are giving just one parameter (s*d*e*f). You have to give four parameters as per your function declaration (s,d,e,f).
Also, please put your code in code tags and state exactly what error you are getting.