LINQ Lambda Group By with Sum

Hi I can do this in method syntax but I'm trying to improve my lambda skills how can I do:

SELECT SUM([job_group_quota]) as 'SUM'
FROM [dbo].[tbl_job_session]
WHERE [job_group_job_number] = @jobnum
and [job_group_ID] like @sess
GROUP BY [job_group_job_number]

I've been messing around with it but can't get it right.

lnq.tbl_job_sessions.GroupBy(a => a.job_group_job_number == jnum)
                    .Select(b => new { b.job_group_quota}).Sum();