暴力解决:

#include <bits/stdc++.h>
using namespace std;
int main(){
	double a,b,c,d;
	cin >> a >> b >> c >> d;
	for(double i = -100.00 ; i <= 100.01 ; i += 0.01) if(round((a * (i * i * i) + b * (i * i) + c * i + d) * 100) / 100 == 0) printf("%.2lf ",i);
	return 0;
}

#include<bits/stdc++.h>
using namespace std;
int main(){
   double a,b,c,d;
   scanf("%lf%lf%lf%lf",&a,&b,&c,&d);
   for(double i =- 100;i <= 100;i += 0.001){
      double j = i + 0.001;
      double y1 = a * i * i * i + b * i * i + c * i + d;
      double y2 = a * j * j * j + b * j * j + c * j + d;
      if(y1 >= 0 && y2 <= 0 || y1 <= 0 && y2 >= 0){
         double x = (i + j) / 2;
         printf("%.2lf ",x);
      }
   }
	return 0;
}