HDU - 2899 Strange fuction

#include <bits/stdc++.h>
#define exp 0.00001
using namespace std;
typedef long long ll;

double solve(double x, double y)
{
    return 42* pow(x, 6) + 48*pow(x, 5) + 21*x*x+ 10*x - y;
}
int main()
{
    double y;
    int t;
    cin >> t;
    while(t--)
    {
        cin >> y;
        double l = 0, r = 100, x0;
        while(l - r <= exp)
        {
            double mid = (l + r)/2;
            if(solve(mid, y) >= 1e-9) r = mid - exp, x0 = mid;
            else l = mid + exp;
        }
        cout << fixed << setprecision(4) << 6*pow(x0, 7)+ 8*pow(x0, 6)+ 7*x0*x0*x0 + 5*x0*x0 - y*x0 + 1e-9 << endl;

    }
    return 0;
}