-- scanf("%f", f);
++ scanf("%f", &f);
And you are performing your operations before getting the input from the user. Should be
#include <stdio.h>
int main()
{
float f, answer;
printf("fahrenheit =");
scanf("%f", f);
answer= 5.0 / 9.0 * (f-32.0);
printf("\n");
printf("%.1f to celsius is %.1f\n", f, answer);
return 0;
}