double *kinji(double x1, double y1, double x2, double y2, double x3) {
double *y3;
y3 = (double *)malloc(sizeof(double));
if (y3 == NULL)
fprintf(stderr, "error: out of memory.\n"), exit(1);
if ((x1-x2) == 0)
fprintf(stderr, "error: divide by zero.\n"), exit(1);
*y3 = ((y1-y2)*x3+(x1*y2-x2*y1))/(x1-x2);
return y3;
}