Just take a bus directly from the isbt Kashmir gate.
or another option is to take metro till Bahadurgarh bus stand and from itself there you get as many buse to Rohtak. Well the metro to Bahadurgarh saves your time .
There is no option of going Rohtak through the Metro.
You can take Bus, Cab, and Train.
Or you can also consider the option of the bla-bla cab it would be more convenient to you.