Threads quite clear and there is nothing to discuss more. Thing you have to do is remember the formula for the circumference and area of ​​a circle. I have the following formula:

P = 2πR
S = πR²

So the formula has. Now we just code formula.

/*
*	Calculator P and S of Circle
*/

#include <stdio.h>
#include <math.h>

int main() {
	float r, p, s;

	printf("Enter r = ");
	scanf("%f", &r);

	p = 2 * M_PI * r;
	s = M_PI * r * r;
	printf("Perimeter of circle = %f\n", p );
	printf("Acreage of circle = %f\n", s );

	return 0;
}

In the code above, you notice yourself using constant M_PI available in the library math.h to be high precision.

From all the above, you do the following exercise.