for example link math lib
gcc main.c -lm
man pow

POW(3)                                                                                            Linux Programmer's Manual                                                                                            POW(3)

NAME
	   pow, powf, powl - power functions

SYNOPSIS
	   #include <math.h>

	   double pow(double x, double y);
	   float powf(float x, float y);
	   long double powl(long double x, long double y);

	   Link with -lm.

   Feature Test Macro Requirements for glibc (see feature_test_macros(7)):

	   powf(), powl():
		   _BSD_SOURCE || _SVID_SOURCE || _XOPEN_SOURCE >= 600 || _ISOC99_SOURCE || _POSIX_C_SOURCE >= 200112L;
		   or cc -std=c99

DESCRIPTION
	   These functions return the value of x raised to the power of y.