例えば >>223 のソースならこんな風にさ。

#include <stdio.h>
#include <math.h>
#define pai 3.14

typedef struct {
float x;
float y;
} point;

typedef struct {
float a00,a01,a10,a11;
} matrix;

void asdf(point pnt[3], matrix mat , point res[3]){
int i;
for(i=0;i<3;i++){
res[i].x = mat.a00*pnt[i].x+mat.a01*pnt[i].y;
res[i].y = mat.a10*pnt[i].x+mat.a11*pnt[i].y;
}
}

int main(void){
int i;
float theta,phi;
matrix mat;
point pnt[3];
point res[3];
for(i=0;i<3;i++){
printf("%d個目の点を入力してください\n x = ",i+1);
scanf("%f",&pnt[i].x);
printf(" y = ");
scanf("%f",&pnt[i].y);
}
printf("θ = ");
scanf("%f",&theta);
phi=(pai*theta)/180.0;
mat.a00 = cos(2*phi);
mat.a01 = sin(2*phi);
mat.a10 = sin(2*phi);
mat.a11 = -cos(2*phi);

asdf(pnt,mat,res);
for(i=0;i<3;i++){
printf("(%10f,%10f)→(%10f,%10f)\n",
pnt[i].x, pnt[i].y, res[i].x, res[i].y );
}

return 0;
}