C :

#include<stdio.h>

int main()
{
	int t;
	float xp1,xp2,xp3,yp1,yp2,yp3;
	double a,k,b,s;
	scanf("%d",&t);
	while(t--)
	{
		scanf("%f%f%f%f%f%f",&xp1,&yp1,&xp2,&yp2,&xp3,&yp3);
		a=(yp2-yp1)/(xp2-xp1)/(xp2-xp1);
		k=(yp2-yp3)/(xp2-xp3);
		b=yp2-(yp2-yp3)/(xp2-xp3)*xp2;
		s=a/3*xp3*xp3*xp3+(-2*a*xp1)/2*xp3*xp3+(a*xp1*xp1+yp1)*xp3-k/2*xp3*xp3-b*xp3-(a/3*xp2*xp2*xp2+(-2*a*xp1)/2*xp2*xp2+(a*xp1*xp1+yp1)*xp2-k/2*xp2*xp2-b*xp2);
		printf("%.2lf\n",s);
	}
	return 0;
}

C++ :

#include<stdio.h>

int main()
{
	int t;
	float xp1,xp2,xp3,yp1,yp2,yp3;
	double a,k,b,s;
	scanf("%d",&t);
	while(t--)
	{
		scanf("%f%f%f%f%f%f",&xp1,&yp1,&xp2,&yp2,&xp3,&yp3);
		a=(yp2-yp1)/(xp2-xp1)/(xp2-xp1);
		k=(yp2-yp3)/(xp2-xp3);
		b=yp2-(yp2-yp3)/(xp2-xp3)*xp2;
		s=a/3*xp3*xp3*xp3+(-2*a*xp1)/2*xp3*xp3+(a*xp1*xp1+yp1)*xp3-k/2*xp3*xp3-b*xp3-(a/3*xp2*xp2*xp2+(-2*a*xp1)/2*xp2*xp2+(a*xp1*xp1+yp1)*xp2-k/2*xp2*xp2-b*xp2);
		printf("%.2lf\n",s);
	}
	return 0;
}