The range is the highest number minus the lowest number. In this case, the highest number is 10 and the lowest number is 6, so the range is 10-6=4
To find the students in the group, simply add up the frequency from each mark. I'll give you an example solution, but I challenge you to do this on your own!
For example, say that there are 4 students with 3 marks and 6 students with 4 marks. There are then 4+6=10 total students in the group
To find the mean, simply add up all up the marks with the amount of frequencies and divide it by the amount of students.
Take our example from earlier - we can write it as a list. Since there are 4 students with 3 marks, we have 3, 3, 3, 3 ( 3 four times) Since there are 6 students with 4 marks, we have 4, 4, 4, 4, 4, 4 (4 six times)
Add them all up, and divide them by the total amount of students (10) to get (3+3+3+3+4+4+4+4+4+4)/10=(3*4+4*6)/10=36/10=3.6
Good luck, and feel free to ask questions if you're confused!