E6420 has 2 graphics cards?

The Intel 3000 is basic and integrated on the board and the Nvidia 4200m is an optional add on. Check the Device Manager to see what is installed.

My laptop dell latitude e6420 only show Intel 3000 its not showing nvidia card 

That's the configuration most of these will have shipped with.  Only some higher-line models will have the optional nVidia GPU (most of these were sold to businesses having little need for a relatively expensive GPU option).

Mine too, I have determined that the Intel graphics are tied to the display while the Nvidia is tied to the HDMI out on the back right corner, which is kind of a shame