CODE:

#include 
#include 

typedef struct {
	float x, y;
} vec2;

void calc(vec2 a1, vec2 a2, float power, vec2* b1, vec2* b2)
{
	float length;
	vec2 a3 = { 
		-(a2.y - a1.y),
		a2.x - a1.x
	};
	
	length = hypot(a3.x, a3.y);
	a3.x /= length;
	a3.y /= length;
	
	b1->x = a2.x + a3.x * power;
	b1->y = a2.y + a3.y * power;
	b2->x = a2.x + a3.x * -power;
	b2->y = a2.y + a3.y * -power;
}

int main(void) {
	vec2 a1 = { 1.0f, 0.0f };
	vec2 a2 = { 1.0f, 2.0f };
	
	vec2 b1, b2;
	calc(a1, a2, 3.0f, &b1, &b2);
	printf("(%f, %f), (%f, %f)", b1.x, b1.y, b2.x, b2.y);
	
	return 0;
}