I'm not 100% sure of the best way to concatenate strings in Objective-C, but the idea is, assuming you have a variable countryId
with the id you're looking for, you would do something like this:
NSString *xpath =
[NSString stringWithFormat:@"%@/%@/%@",
@"/countries/country[@id = '", countryId, @"']/subsidiaries/subsidiary"];
resultNodes = [rssParser2 nodesForXPath:xpath error:nil];