var q = from f in first
        join m in model on f.Id equals m.fID
        where m.year >= 2014 && m.year <= 2020
        group new { f, m } by m.year into g
        select new 
        { 
           year    = g.Key,
           credit1 = g.Sum(x => x.f.Acres * x.m.credit1),
           credit2 = g.Sum(x => x.f.Acres * x.m.credit2),
           credit3 = g.Sum(x => x.f.Acres * x.m.credit3)
        };