I need to find the computers which are applicable to particular baseline and particular custom sites. Could anyone help me on this?
You haven’t mentioned what you need it for but if it just for a list, I’d use the default functionality in both console (“Applicable computers” tab in the baseline) or in Web Reports (just find the baseline via “Explore Computers” -> Content and click on the baseline, it will have all the details of both “still relevant” and “remediated” machines). If it is for some kind of API-based data retrieval, something like this should do:
names of applicable computers of bes fixlets whose (baseline flag of it and (name of it = "BaselineName") and (display name of site of it = "SiteName"))