Ubuntu 14.04 Nvidia driver fail

i have bought a new Dell inspiron 15 - 3542 that has Nvidia (GM108M [GeForce 840M]). every time i install the Nvidia driver i have a black screen at the next boot, so i need to format again ! which causes me a huge pain ! this is what i get from (sudo lshw -C video)

 *-display               
   description: VGA compatible controller
   product: Haswell-ULT Integrated Graphics Controller
   vendor: Intel Corporation
   physical id: 2
   bus info: pci@0000:00:02.0

   version: 0b
   width: 64 bits
   clock: 33MHz
   capabilities: msi pm vga_controller bus_master cap_list rom
   configuration: driver=i915 latency=0
   resources: irq:60 memory:f7400000-f77fffff memory:d0000000-dfffffff     ioport:f000(size=64)


*-display UNCLAIMED
       description: 3D controller

       product: GM108M [GeForce 840M]
       vendor: NVIDIA Corporation
       physical id: 0
       bus info: pci@0000:08:00.0
       version: a2
       width: 64 bits
       clock: 33MHz
       capabilities: pm msi pciexpress cap_list
       configuration: latency=0
       resources: memory:f6000000-f6ffffff memory:e0000000-efffffff memory:f0000000-f1ffffff ioport:d000(size=128) memory:f7000000-f707ffff

And this is what i get when i run lspci | egrep 'VGA|3D'

00:02.0 VGA compatible controller: Intel Corporation Haswell-ULT Integrated Graphics Controller (rev 0b)
08:00.0 3D controller: NVIDIA Corporation GM108M [GeForce 840M] (rev a2)

Any idea how to get this Nvidia card work without getting the black screen again? Please help me this is urgent !! i appreciate it !

Finally i could figure this out !!

the answer is here http://www.noobslab.com/2014/12/use-nvidia-graphics-drivers-in.html

Thank you all for participating !! Love you all !