According to the question,
Time taken for both of them to do the work = 3hrs
Time taken for Ramesh to do the work = 4hrs
So, when both of then work together, both of them do half the work. Therefore, Ramesh does his half in two hours while Suresh does the work at the same time in three hours. (As both work together, both the time over lapses and the work is done in three hours)
Thus, we get that Suresh does half his work in 3 hrs so he can complete the work alone in 6 hours.
Hope that helps !!