I just bought Acer Monitor

Question

I just bought Acer Monitor X213W 21.6 inches There are three cords, one for Electrical supply, one for computer attachment, the third wire is for a particular purpose goes one end to the monitor and other end to the socket of Computer.I have Pentium 5 dual core 1.8 GH new computer but did not find the socket which the thirs wire cann be attached. This wire has 2 sets of 9 pin and a small horizontal pin.What should I do now?