hdu 2199 Can you solve this equation?

#include<stdio.h>
#include<math.h>
double f(double x)
{
    return 8*x*x*x*x+7*x*x*x+2*x*x+3*x+6;
}
int main(void)
{
    int t;
    double y,x1,x2,x3,y1,y2,y3;
    scanf("%d",&t);
    while(t--)
    {
        scanf("%lf",&y);
        x1=0;
        x2=100;
        y1=f(x1)-y;
        y2=f(x2)-y;
        if(y1>0||y2<0)
        {
            printf("No solution!\n");
        }
        else
        {
            while(fabs(y1-y2)>=0.0001)
            {
                x3=(x1+x2)/2;
                y3=f(x3)-y;
                if(y3>=0)
                {
                    x2=x3;
                }
                else
                {
                    x1=x3;
                }
                y1=f(x1)-y;
                y2=f(x2)-y;
            }
            printf("%0.4f\n",x3);
        }
    }
    return 0;
}