Hello, you will need Displayport to DVI-D cable such of this one. Please note:  many cables are unidirectional, meaning its signals will travel in one direction only.
Hi, I've had Dells for years.  I have a really old Dell W2300 LCD TV Monitor that I love.  It's survived several moves, including a couple cross country ones.  I had a Lenovo tower recently that went up in smoke so I went back to Dell and bought a refurbed Optiplex 7040 tower this time.  It finally arrived today.  I've been without a desktop for almost three weeks now and really need to get it up and running but...the plug for the monitor doesn't match the plug in for the tower!!!  I think there must be an adaptor out there but I'm not techy enough to know which one I'd need.  Can anyone help me using non-tech terminology?  Thanks so much.   
New Optiplex refurb & Old Monitor