i2c_master error

i dont know why it gives undefined reference error i already attached and defined .c and .h library file and defined them. how i can get rid of this error thank you.

C:\Users\HP\AppData\Local\Temp\cceDEDJc.ltrans0.ltrans.o: In function `sendcommand(unsigned char)':

C:\Users\HP\Documents\Arduino\oled/oled.ino:199: undefined reference to `i2c_start(unsigned char)'

C:\Users\HP\AppData\Local\Temp\cceDEDJc.ltrans0.ltrans.o: In function `senddata':

C:\Users\HP\Documents\Arduino\oled/oled.ino:207: undefined reference to `i2c_start(unsigned char)'

collect2.exe: error: ld returned 1 exit status

exit status 1
Error compiling for board Arduino/Genuino Uno.

Please post your full sketch.

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