How would I make a new function, shrink(float x), that would divide the input value by 2?

How would I make a new function, shrink(float x), that would divide the input value by 2?

#include <stdio.h>

void printHelp () {
  printf ("\n");
  printf ("a: a(x) = x*x\n");
  printf ("b: b(x) = x*x*x\n");
  printf ("c: c(x) = x^2 + 2*x + 7\n");
  printf ("q: quit\n");
}

void a(float x) {
  float v = x*x;
  printf ("  a(%.2f) = %.2f^2 = %.2f\n", x, x, v);
} // end function a

void b(float x) {
  float v = x*x*x;
  printf ("  b(%.2f) = %.2f^3 = %.2f\n", x, x, v);
} // end function b

void c(float x) {
  float v = x*x + 2*x + 7;
  printf ("  c(%.2f) = %.2f^2 + 2*%.2f + 7 = %.2f\n",
            x, x, x, v);
} // end function c

int menu () {
  char selection;
  float x;
  printHelp ();
  scanf ("%s", &selection);
  if (selection == 'q') return 1;
  scanf ("%f", &x);
  if (selection == 'a') a(x);
  if (selection == 'b') b(x);
  if (selection == 'c') c(x);
  return 0;
} // end function menu

int main() {
  while (menu() == 0);
  printf ("... bye ...\n");
  return 0;
} // end main

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