Q: 2012 air to 34" Dell ultrawide monitor - won't stay on; auto into sleepmode

I have a 2012 macbook air to 34" dell ultrawide monitor. When i hook it up,  the screen flickers on then off immediately going into sleep mode. Have tried trouble shooting with mac genius bar, have tried multiple hook ups hdmi and adaptors. Monitor works with newer computers but rather than getting a new computer, i would just return the dell and get a different monitor, unless there is a fix for my air .... ? Mac genius suggested posting here! Thanx.

Dell has no software drivers for Apple products and no support for macs.

Thinking of updating system ... at store now to see if booting this system with sierra will work. ..all ideas helpful, thanx!